When evolutionary-minded medicine gets it (possibly) wrong about childbirth interventions

No one is saying that medicine isn't brilliant and hasn't saved lives. But it does intervene more than necessary when it comes to pregnancy and childbirth.

Part of that unnecessary intervention is driven by lack of experience. Part is an economically-driven disrespect for time. (Give childbirth some motherlovin' time.) Another part, related very much to experience, is how difficult it is to decide when intervention is and isn't necessary, especially when things are heating up. But another part of the trouble actually lies in the evolutionary perspective. Unfortunately it's not all rainbows and unicorns when M.D.s embrace evolution. Instead, evolutionary thinking is biasing some medical professionals into believing that, for example, birth by surgical caesarean is an "evolutionary imperative."

Here's one recent example in The American Journal of Obstetrics & Gynecology of how the evolutionary perspective is (mis)guiding arguments for increased medical intervention in childbirth.

link to paper

It's a fairly straight-forward study of over 22,000 birth records at a hospital in Jerusalem. The authors ask whether birth weight (BW) or head circumference (HC) is more of a driver of childbirth interventions (instrumental delivery and unplanned caesareans) than the other. Of course, the focus is on the biggest babies with the biggest heads causing all the trouble, so the authors narrow the data down to the 95th percentile for both. Presumably they're asking this question about BW and HC because both can be estimated with prenatal screening. So there's the hope of improving delivery outcomes here. And, of course, the reason they ask whether head size or body mass is more of a problem is because of evolution. They anticipate that they'll discover that heads are a bigger problem than bodies because of the well-known "obstetrical dilemma" (OD) hypothesis in anthropology.

OD thinking goes like this: Big heads and small birth canals are adaptive for our species' cognition and locomotion, respectively, but the two traits cause a problem at birth, which is not only difficult but results in our species' peculiar brand of useless babies. (But see and see.)

So, since we're on the OD train, it's no surprise when we read how the authors demonstrate and, thus, conclude that indeed HC (head circumference) is more strongly associated with childbirth interventions than BW (birth weight), at least when we're up in the 95th percentile of BW and HC. Okay.

They use this finding to advocate for prenatal estimation of head size to prepare for any difficulties a mother and her fetus may be facing soon. Okay.

Sounds good. Sounds really good if you support healthy moms and babies. But it also sounds really good if you already see these risks to childbirth through the lens of the "obstetrical dilemma" with that OD thinking helping you to support "the evolutionary imperative" of the c-section. Okay.

Too many "Okays" you're thinking? You're right. There's a catch.

When you dig into the paper you see that "large HC" heads are usually about an inch (~ 2.5 cm) greater in circumference than "normal HC" ones. (Nevermind that we chopped up a continuum of quantitative variation to put heads in arbitrary categories for statistical analysis.) And when you calculate the head diameter based on the head circumference, there is less than 1 cm difference between "large" and "normal" neonatal heads in diameter. That doesn't seem like a whole lot considering how women's bony pelvic dimensions can vary more than that.  Still, these data suggest that the difference between a  relatively low risk of having a c-section and a relatively high risk of having a c-section amounts to less than a centimeter in fetal head diameter. And maybe it does. Nobody's saying that big heads aren't a major problem sometimes! But maybe there's something else to consider that the paper absolutely didn't.

Neonatal heads get squeezed and molded into interesting shapes in the birth canal.

The data say that normal HC babies get born vaginally more often than large HC ones. But this is based on the head measures of babies who are already born! If we're pitting head circumference (HC) of babies plucked from the uterus against the HC of babies who've been through hello! then of course the vaginally delivered ones could have smaller HCs.

C-sected babies tend to have rounder heads than the ones squeezed by the birth canal. It's impossible to know but I'm fairly confident about this, at least for a subsample of a population: Birth the same baby from the same mother both ways, vaginally and surgically, and its head after c-section will have a larger HC than its squeezed conehead will after natural birth.

Measuring newborn head circumference (HC). source

When we're talking about roughly 2.5 cm difference in circumference or less than 1 cm difference in diameter, then I'd say it's possible that neonatal cranial plasticity is mucking up these data; we're sending c-sected babies over into the "large HC" part of the story just because they were c-sected in the first place. So without accounting for this phenomenon, the claim that large head circumference is more of a cause of birth intervention, of unplanned c-sections, than large body mass isn't as believable.

If these thoughts about neonatal cranial molding are worthwhile, then here we have a seemingly useful and very high-profile professional study, grounded in the popular but deeply flawed obstetrical dilemma hypothesis, that is arguing for medical intervention in childbirth based solely on the difference in head size measures induced by those very medical interventions. 

The circle of life!

'Obstetric dilemma' skeptic has c-section and remains skeptical ... & ... Why my c-section was natural childbirth

This is a new kind of Tale for me. The rock'n'roll's turned way up, and every couple sentences I have to stop typing to twirl a blue hound dog, a bear holding an umbrella, a Flying Spaghetti Monster, and other oddities that I strung up to hypnotize this little guy into letting me type one thought at a time:

The thing that needs to be hypnotized.
Or the three wise monkeys say: The thing that makes it impossible to create or to dwell on the negative. (e.g. his birth by c-section)

That young primate's the reason I've been quiet for a while here on the MT. And he's the reason I'm a bit more emotional and I cry harder than usual at Rise of the Planet of the Apes (those poor apes!), Cujo (that poor dog!), and other tearjerkers. But he's also the reason my new favorite animal is plain old, fascinating, and dropdead adorable Homo sapiens.

In anthropological terms, he's the reason I'm overwhelmed, not just in love but in new thinking and new questions about the evolution of human life history and reproduction, and then what culture's got to do with it and with our reconstruction of it.

Some context would help, probably.

For the past few years I've been challenging the 'obstetric dilemma' hypothesis--the idea that hominin mothers' bipedal pelves have shortened our species' gestation length and caused infant helplessness, and that antagonistic selection between big-brained babies and constrained bipedal mothers' pelves explains childbirth difficulty too.

[For background see here or here or here or here.]

As part of all that, I've been arguing that the historically recent surge of c-sections and our misguided assumptions about childbirth difficulty and mortality have muddled our thinking about human evolution.

So, once I was pregnant, you might imagine how anxious I was to experience labor and childbirth for myself, to feel what the onset of labor was like, and to feel that notorious "crunch" that is our species's particular brand of childbirth. Luckily I was not anxious about much else the future might hold because modern medicine, paid for by my ample health insurance, would always be there to make it all okay. After a long pregnancy that I didn't enjoy (and am astonished by people who do) I was very much looking forward to experiencing childbirth. In the end, however, my labor was induced and I had a bleeping c-section.

But my bleeping c-section's only worth cussing over for academic reasons because the outcome has been marvelous, and the experience itself was out of this world.

We'll get to the reasons for my c-section in a second, but before that, here are the not-reasons...

First of all, I did not have a c-section because I fell out of a tree with a full bladder.

Second of all, shut your mouth... a c-section was not inevitable because of my hips.

Okay, you got me. I've never been even remotely described as built for babymaking. My hips are only eye-catching in their asymmetry. One side flares out. It might be because when I was 15 years old I walked bent-kneed for a few months pre- and post-ACL reconstruction. That leg's iliac crest may have formed differently under those abnormal forces because, at 15, it probably wasn't fused and done growing yet. If you like thinking in paleoanthropological terms like I do, then my left side is so Lucy.

Anyway. I'm not wide-hipped. However, guess how many nurses, doctors, or midwives who were involved in our baby's birth think my pelvis was a note-worthy factor in my c-section? Not one.

Hips do lie! Inside mine there's plenty of room to birth a large baby. Two independent pelvic exams from different midwives (who knew nothing of my research interests at the time) told me so, and it sounded like routine news to boot. Although one midwife asked me "do you wear size nine and a half shoes?" (no, I wear 8) which was her way of saying, "Girl, you're running a big-and-tall business. You got this."

What you probably know from being alive and knowing other people who were also born and who are alive (or what you might hear if you ask a health professional in the childbirth biz) is that most women are able to birth babies vaginally, even larger-than-average babies. And that goes for most women who have ever lived. Today, "most women" includes many who have c-sections because not all c-sections are performed because of tight fit between mother's birth canal and baby's size. As I understand it, once the kid's started down into the birth canal and gets stuck, a c-section's no longer in the cards. So performing c-sections for tight fit is a preventative measure based on a probability, not a reflection of an actual tight fit. In the mid 20th century, tight fit used to be estimated by x-raying pregnant women and their fetuses. Can you imagine? And this was right about the time the obstetric dilemma hypothesis was born. I don't think that's a coincidence.

Here's a list of reasons for c-sections. Tight fit is included in the first bullet point. Tight fit is one of the few quantifiable childbirth risks. No wonder it's so prominent in our minds. That list excludes "elective" ones which can be done, at least in Rhode Island, if they check the box that says "fear of childbirth". And that's not even close to a list of reasons why women around the world and throughout history have died during or as a result of childbirth. For example, about a hundred years ago women were dying all over the place because of childbed fever.

Anyway, we should assume that I am like most women and expect that I could have given birth the way Mother Nature intended: through my birth canal and with the participation of other humans. Oh yeah, when it comes to humans, social behavior and received knowledge are part of natural childbirth. Even this natural childbirth (which has inspired a forthcoming reality television show featuring women giving birth in the wild!) involves the supportive and beneficial presence of other humans as well as the culture that the mother brings to the experience.

But a c-section's just culture too, so could it be part of "natural" childbirth, then?

I'm inclined to blurt out yes, of course! because I don't support calling anything that humans do "unnatural." But I know that's not something everyone agrees with. It's politics. For example, many of you out there don't flinch an inch at the subtitle of Elizabeth Kolbert's book, "The Sixth Extinction: An Unnatural History."  And given the present energetic movement against childbirth interventions, describing c-sections as "unnatural" as climate change could help minimize unnecessary ones for those who wish to give birth vaginally.

So there we have it. These are the two enormous issues raised by my own little c-section: What can it teach us about the evolution of gestation length, infant helplessness, and childbirth difficulty? And could it be considered natural?

One way for me to get at these questions is to try to understand why I experienced "unnatural" childbirth in the first place. So here goes.

Here's why I think I had to have a c-section:

1. My pregnancy ran into overtime.

This is expected for nulliparous mothers. I visited one of my OBs on my due date. He put his finger on the calendar on the Friday that was two weeks out and joked, "Here's when we all go to jail." Then he asked me, "Who do you want to deliver your baby? I'll see when they're on call before that Friday and schedule your induction then." And I chose my favorite midwife and he scheduled the induction.

All right so I was running late compared to most women, but that's still natural, normal. But it also means risks are ever-increasing by the day. And no matter how small, that the professionals know how to mitigate the biggest risks of all, *deaths*, means that they try to do that. They're on alert already as it is, and then they're even moreso on edge when you're overdue. Especially when it's your first baby and you're a geezer, over 35 years of age.

Now, does going overdue mean the baby keeps growing? Maybe, but not necessarily and not necessarily substantially. Both of us, together, should have been reaching our maximum, metabolically. There's only so much growing a fetus can do inside a mother.

When I approached my due date, and then once I went past it, I tried to eat fewer sweets to make it less comfortable in my womb. I also went back to taking long, hard walks, five milers, even though it was hard on my bladder because I thought that might help kick him out too. I even ran the last of my five miles the day before my induction, to no avail other than the mood boost it gave me.

2. I didn't go into labor naturally by my due date or by my induction date 11 days later. 

Although my cervix was ripening, when I went in to be induced I was only dilated 0-1 cm. I had 9+ more to go before the kid could get out at 10. So a balloon catheter was inserted and filled with water, and I had to tug on the tail of it, which tugged the balloon, which put pressure on the cervix. It dilated enough that it fell out several hours into the process, and by morning I was dilated 3-4 cm. This was exactly the goal of the catheter, this many centimeters. All was going well. However, that the cervix did not open on its own is already a missing piece of going "natural," of having my own biology contribute to my childbirth experience. So starting this way is already derailing things, making it difficult for anything natural to follow, naturally.

3. The fetus's head was facing the hard way: sunnyside up.

This was assessed by the midwife and cradling my belly in a bedsheet, with me on all fours, she and I could not twist him into a better position. His head, she said, was probably why I did not dilate naturally. When I asked an OB during my postpartum check-up, "What dilates the cervix?", he said "We don't know. But I can tell you it's not with the head like it's a battering ram." Well, then... hmph. And then I asked him if women carrying breech fetuses have trouble dilating their cervixes, or going into labor naturally, and he said not necessarily. No. Hmph.

Regardless of what causes cervical dilation, if the head isn't facing the right direction, it's notoriously tough to get down into the birth canal, let alone through the birth canal. It's not impossible, not even close. But it's not looking good at this point either. Perhaps the contractions will jossle his head into a better position, they said. And the contractions should further dilate the cervix.

4. Contractions didn't get underway, naturally, after the catheter dilation, so the drug pitocin was used. 

Induction and pitocin increase the chances that a mother will ask for drugs to help with pain and that she will have interventions, like a c-section. See for example this paper. What the causes are, I'm not sure. But pointing out the correlation is useful at this point because at this point, without even getting into hard labor yet, and without finding out whether my cervix does its job, I'm more likely than ever to be going to the operating room.

5. After six hours of easy labor and five hours of intense labor, my cervix never dilated past 5 cm.  

It needs to get to 10 cm to get the baby moving into the birth canal. Just like with due dates, I think that blanketly assigning this number to all women is probably not consistent with variable biology, but it's how it's currently done. And maybe any higher resolution, like "Sally's cervix needs to hit 9.7 cm", is pointless.

After several hours pitocin-induced contractions--which at first felt like the no-big-deal Braxton-Hicks ones I'd been having numerous times daily for the whole third trimester--I only dilated 1 cm more. That's even when they upped the pitocin to make them more intense.

But after they saw I'd made essentially no progress and that I was napping to save my energy for when things got bad, they woke me up and broke my bag. It would be nice if they could have let my labor progress slowly, if that's what my body wanted to do, but remember, my personal biology went out the window as soon as induction began. And then when that amniotic fluid oozed out of me, that's when bleep got real.

Every two minutes and then every one and a half, I grabbed Kevin's extended hand and breathed like an angry buffalo humping a locomotive. It was the worst pain of my life and I was afraid I'd never last to 10 cm, so I took the stadol when I told the nurse my pain was now at a 9 out of 10 (all previous answers to this question were no higher than 2). I was going to avoid the epidural no matter what, even at this point, because I was more afraid of the needle sticking out of my spine for hours of labor than I was afraid of these contractions. I have no idea if the stadol dulled any pain, because the pain just got worse, but it did help psychologically because it put me to sleep between contractions. There was no waiting with anxiety for the next one and time flew by. But after five hours of this, I had not dilated any more. But I had vomited plenty! And although I'd fended off the acupuncture (FFS!), I folded weakly and, for the peace of mind of a wonderfully caring nurse, I allowed a volunteer to perform reiki on me. And what a tragedy it was! Wherever she is, there's a good chance she gave up trying to help laboring women, and she may have given up reiki all-together.

The hard labor story ends at five hours because that's about when the nurse actually screamed into the intercom for the doctor. My contractions were sending the fetus into distress.

6. After five hours of intense labor, the fetus was experiencing "distress" at every contraction, as interpreted from his heart-rate monitor. 

Basically, he was bottoming out to a scary heart-rate and only very slowly coming back to a healthy heart-rate just in time to get nailed by another contraction. By the way, this is the official reason listed in my medical records for my c-section: fetal distress.

I know that a heart-rate monitor on the fetus is another one of those medical practices that increases the chances of an "unnatural" childbirth. That's probably because all fetuses are distressed during labor, but observing the horror, and then guessing whether it's safe to let it continue is seemingly impossible. So at some point, like with me and my fetus, they get alarmed and then how do you back down from that?  They gave me an oxygen mask which immediately helped the fetus a bit, but like I said, hackles were already up at this point. Soon thereafter we had a talk with the doctor about how I  could go several more hours like this and get absolutely nowhere with my cervix, and then there are those life and death matters. She never said c-section. I had to eek out between contractions, "So are you saying we need to perform a c-section?" and she said yes, and urgently. A c-section sounded like the only solution at this point both to battered, old me, to clear-minded Kevin, and clearly to the delivery team (and in hindsight, it still does to Kevin and me). Then, lickety-split, the anaesthesiologist arrived, got acquainted with our situation, and made me vomit more. And then like a whirlwind, Kevin's putting on scrubs, and we're told to kiss, and I'm jokingly protesting "I'm a doctor too!" while being wheeled into the operating room because I cannot walk through my contractions.

It's bright white, just like Monty Python said it would be. I sat on the crucifix-shaped operating table to receive all the numbing and pain killing agents through my spine. Somehow they pulled this off while I was still having massive contractions. Then I laid down, arms splayed out to the side, and they drew a curtain across my chest, a nurse told me how creepy it was about to be, and they got to work.

Although the c-section wasn't painful, I could feel everything. This was my childbirth experience. I felt the incision as if she was simply running her finger across my belly, and I felt the tugging and the pressure lifting from my back as they extracted my baby from me. After that, and after I got a short glimpse of him dangly over my left arm--"He's beautiful! He's perfect! He's got a dimple! He growled!"--I continued to feel many things, probably the birth of my placenta, etc...

But I didn't know what exactly I was feeling until I watched a video of a c-section on YouTube. Kevin helped fill in the details too. He had caught a naughty glimpse of the afterbirth scene before being chased back to his designated OR spot with the baby. Thanks to him (and that video) I know now that I was feeling my enormous muscular uterus and some of my intestines being yanked completely out of a small hole right above my pubic bones and then stuffed back in. For a few moments, it must have looked like I was getting re-inseminated by a red octopus.

I tell everyone that it was like going to outer space to give birth. And this, if you know me, is an exciting idea so my eyes are smiling and I sound dreamy when I say "it was like going to outer space to give birth!" I bet you're thinking it's the Prometheus influence, but you'd have the wrong movie. The correct one is Enemy Mine. And it's much more than that, actually. I was as jaw-dropped and awe-struck by humanity during my childbirth experience as I am by space exploration. The orchestration, the specialization, the patience, the years of study, of planning, the calculations, the dexterity. To boldly go. Wow. Like I said, humans are my new favorite animal.

I was back in our little room quicker than most pizza deliveries, where our bright red new baby was trying hard to nurse from his daddy. Then he nursed from me. And the story's all mushy weepy cuddly stuff from now on. So let's not. Let's remember what we're here for. Okay. Right.

7. The cord was wrapped twice around his neck. 

We found this out when he was cut out of me. That didn't help with moving him around in utero to a good position, nor did it help with oxygen flow during contractions! This would not have inhibited his safe vaginal birth, however, at least not necessarily.

8. He was enormous. His head was enormous too. 

He came out a whopping 9 pounds, 13 ounces, 22.25 inches long, with a head circumference of 15.5 inches. They say that's heavier than he'd be if born vaginally because he didn't get all the fluids squeezed out of him. But still, that's large. According to the CDC he was born as heavy as an average 3.5 month-old boy. His head was about the size of an average 2.5 month-old.

Red line is our baby's head circumference at birth. (source)

Way back at the mid-pregnancy ultra-sound, we knew he was going to be something. And then if you'd seen me by the end, like on my due date, you might have guessed I was carrying twins. I was so big that my mom joked she thought maybe a second fetus was hiding behind the other one, undetected.

Smiling on my due date because pregnancy was almost over. 
(By the way, I could still jog and I dressed weird while my body was weird.)

If I hadn't had the means to eat so much like I did during pregnancy, perhaps he wouldn't have grown so large inside me. If I hadn't lived such a relaxed lifestyle while pregnant, maybe he wouldn't have grown so large inside me. If I didn't have a medical safety net waiting for us at the end, perhaps I would have been scared into curbing my appetite from the get go. I gained 40 pounds. With this body, but in a different life, a different place, a different time, maybe I wouldn't have. Probably I wouldn't have.

His size has got to have influenced a few of those other contributors to my c-section. But clearly it's more complicated than his size. And this brings us back to the obstetric dilemma. Let's say he was too big or that his large size screwed everything up, even if he could technically fit through the birth canal. Well then, why didn't I go into labor? Labor triggers are, to me, a significant problem when it comes to explaining the evolution of gestation length in humans, and whether we have a unique problem at the end.

If our pregnancy length is determined by available energy, energy use, and metabolism (here and here) then women like me who go overdue, who are clearly not killing our babies inside us either, are just ... able to do that. But doing that clearly leads to problems in our species (one of the few known) that has such a tight fit to begin with.

If our pregnancy length is determined by our birth canal size, and any anatomical correlates, then why didn't I go into labor before my fetus got so big? What went wrong? What's frustrating too is, for my n of 1, we'll never know if I could have given birth vaginally because I never got the chance to try.

These seem like simple questions but they are deceptively complex. And I think there will be some exciting discoveries to come from medicine and anthropology in the coming decades to explain just how our reproduction works which will in turn help us reconstruct how it evolved.

What's my birth experience got to do with evolution? Why, everything. It's got everything to do with evolution, because if it's not evolution, it's magic.  And that's kind of where I'm coming from when I say that my c-section was still natural childbirth. It wasn't unnatural and it certainly wasn't supernatural. Sure, it's politics. I'm invested in the perspective that humans are part of the evolving, natural world and want others to see it that way or, simply, to understand how so many of us see it that way. But it's not just evolution that's got me enveloping culture into nature and that's got me all soft on the folks who drive fancy cars who cut my baby out of me.

Who knows what could have happened to my son or to me if we didn't have these people who know how to minimize the chances of our death? It's absolutely human to accumulate knowledge, like my nurses, midwives and doctors have about childbirth. Once learned, it's difficult for that knowledge to be unseen, unheard, unspoken, unknown. Why should we expect them to throw all that away so that we can experience some form of human being prior to that knowledge?

Nature vs. Culture? That's the wrong battle.
What matters is which one can fight hardest on my behalf against the unthinkable.

Maybe childbirth is so difficult because it can be. We've got all this culture to help out when things get dicey, with or without surgeons. On that note, maybe babies are so helpless because they can be. We've got all the anatomy and cognition to care for them and although the experiment would be impossible, it's doubtful any other species but ours could keep a human baby alive for very long. It could just be our dexterous hands and arms, but it could be so much more, like awareness of their vulnerability and their mortality,and (my favorite pet idea) awareness that they're related to us. Culture births and keeps human children alive with or without obstetricians. It's in our nature. Maybe it's time we let all this culture, our fundamental nature, extend into the operating room.

You keep citing our paper. I don't think it means what you think it means.

Quote here
(image: princessbride.wikia.com)

I haven't published much. Activity in my scholar.google profile isn't so hectic and exponential that I can't dig into things a little more than your more prolific scholar.

So as I continue the line of research kicked off by our paper (Metabolic hypothesis for human altriciality), I naturally want to read the articles that cite it. Scholar.google is great for showing me those. And while I dig around, I also have the opportunity to see why these authors cited our paper.

For background, here's a short synopsis of our paper that I'm Google-stalking:
Humans are thought to be born when we're born to escape the bipedally-adapted, gestation-constraining birth canal ("obstetrical dilemma"), but it's more likely that we're born when we're born because that's all the fetal tissue our mother can grow ("EGG: energetics of gestation and growth"). 

That's it. There's a lot more in the paper but that's the gist. Now, how's it being cited?

Let's go paper by paper. Don't worry. There are only five.

***

Increased morphological asymmetry, evolvability and plasticity in human brain evolution
Our paper is #69 in this one.Here's where it's cited:

"Biomechanical [68] or metabolic [69] constraints causing human altriciality may have provided another key preadaptation for the evolution of modern human cognition by allowing an increased period of postnatal modelling of the developing brain via the interaction with complex social and cultural environments [70]."

Considering the results of energetic limits and metabolic constraints to be adaptive ranks pretty far up there on the Panglossian scale. Regardless, this is a perfectly good citation of our paper. Thanks to the authors for not citing us later in the sentence.

Good? Yes. 

*** 

Global Geometric Morphometric Analyses of the Human Pelvis Reveal Substantial Neutral Population History Effects, Even across Sexes

Our paper is #90 in this one. (OoA = Out of Africa) Here's where it's cited:

"Our analyses testing for obstetrical constraints in shape variation indicated no difference in the neutral OoA pattern between males and females, a difference that would be expected if constraints were stronger in females than in males. This result is consistent with Tague’s [51] finding that males are not necessarily more variable in pelvic morphology than females. It is also consistent with recent suggestions that the obstetric dilemma may be influenced more by maternal energetics than pelvic morphology per se [90,91]."

Good? Yes.

***

Many ways to die, one way to arrive: How selection acts through pregnancy
Our paper is #23 in this one. (GDM = gestational diabetes mellitus.) Here's where it's cited:

"GDM and preeclampsia are common diseases, with grave consequences in pregnancy, and thus may strongly impact upon reproductive fitness. GDM affects 4–20% of pregnancies in different populations worldwide [19]. It can cause macrosomia, in which the fetus grows too large to fit through the maternal pelvis [20–23]."

Here is the only part of our paper that mentions diabetes:

"uncontrolled gestational diabetes is commonly associated with postterm parturition (Langer O, Kozlowski S, Brustman L. 1991. Abnormal growth patterns in diabetes in pregnancy: A longitudinal study. Isr J Med Sci 27:516–523.)"

Notice how our paper talks about postterm parturition and neither fetal size nor fetal size relative to maternal pelvis. Maybe the paper we cite does.

Good? No.

***

Bony pelvic canal size and shape in relation to body proportionality in humans
 Here's where it's cited:

"This “obstetric dilemma” (Washburn, 1960) has recently been questioned on biomechanical (Warrener, 2011; but see Whitcome et al., 2012) and energetic grounds (Dunsworth et al., 2012)."

Good? Yes. Although it could also go under the biomechanical part of that sentence.

***

Teaching the principle of biological optimization
Our paper is #4 in this one. Here's where it's cited:

"It has been hypothesized that the timing of human birth optimizes the ability of cognitive and motor neuronal development in the child by allowing the child to maximize the absorption of important cultural information (memes) in its environment [4]." 

Yes it has, but not by us. Here's what our paper says (by the way, we were forced to address this by reviewers):

"Finally, a fourth possibility, originally proposed by Portmann (5), is that the timing of human birth and degree of neonatal brain development optimizes cognitive and motor neuronal development (50)."

Our paper points to others which should be dug up when possible and, if not, then it should be explicit that the author means to point readers to references within ours, not to ours itself.

Good? No. 

***

Sure it's a small sample size but people aren't as good as they should be at citing our paper.  I know I've made these same mistakes and I know there's no way coauthors and reviewers can catch all of them especially in interdisciplinary works, but I just thought I'd pipe up and go through with this exercise here to remind myself and others to do our homework better.

But beyond a nice little reality check, there's a larger issue...Can I really consider all these citations as meaningful indicators of my research's value? Are citations truly reflecting the impact of my work if they misrepresent it? Should I include this little expose in my portfolio come promotion and tenure time to demonstrate how I should, in reality, have a lower citation count... that I should have fewer points in this game?

Inconceivable!

Here's why I won't be revealing any of this to my evaluators: I'm being compared against others whose citation totals are also padded with misrepresentation. It's only fair that I assume the same advantages that everyone else does. Who cares about all the larger implications for Scholarship, Evidence, and Knowledge when my job is on the line? I need those points.

So I change my tune. I don't care how you cite our paper. Thank you for citing! Cite as you wish.

***

Note: Earlier in 2013 I read The Princess Bride for the first time and YOU ARE MISSING OUT if you haven't.

Taking our (lovely lady) lumps

Of course I'm still talking about our PNAS paper. Wouldn't you, in my shoes?

Since getting the back story, the story, and the implications for your pregnancy out of the way, I can finally talk about some important cultural and philosophical implications of our paper.  

Aw. The poor girl's compromised. 

Are ladies' hips compromised?
Women aren't called broads for nothing. We have, on average, larger dimensions of the pelvis that comprise the birth canal (linked into broader hips) than men do and this is not just relatively but absolutely and this is not just in the U.S., this is species-wide (1).

There is no better explanation for this than it's due to selection for successful childbirth.

But somehow with the combination of classic biomechanical theory, plus obvious performance differences between the sexes, it has become ingrained in our thinking that wider hips make women athletically inferior to men. In this line of thinking, the  male pelvis is the human ideal because it's part of a superior athlete's body. The female pelvis, therefore, is second-rate--compromised for necessary reasons to do with childbirth.

But recent research by Anna Warrener--which she was so generous to contribute to our paper--shows that hip breadth fails to predict the biomechanical values that are used to calculate walking and running economy. There's support, too, from prior studies that used different or less complex models. So the notion that wide hips are worse at walking and running is not supported by current evidence.

He must be the fastest swimmer alive because of his man hips.

Not only does Anna's research call into question whether women's hips are to blame for our failure to dominate sports, but it also lends strength to any doubt that slightly wider hips that could better accommodate a neonate or that could birth a more developed neonate (i.e. one that is more precocial like all the other primates) are being selected against in favor of proper walking and running ability.

Of course, there could be alternative explanations. Selection could be keeping wide hips from getting wider because of some yet to be understood horrible side effect of too-wide hips. It could be that wider hips than we have now would increase the stress on the hips, knees and ankles to the point of immobility. It could mean that the soft tissues of the pelvic floor would be stressed beyond their mechanical properties and strained to failure. It's also possible that pregnancy itself requires a narrow-enough pelvis to carry the fetus above it, and that getting any wider down there in the swimsuit area would mean the fetus literally falls out before it's ready. But to my knowledge, we don't have good understandings of any of this, at least not in evolutionary terms.

Given research like Anna's, it's much harder to support this idea that we're at the perfect balance now in the ladies' pelvis: With pelvic width (despite all the variation) thanks to childbirth keeping it perfectly wide but bipedalism keeping it perfectly narrow.

But that's--as I'm finding more and more with this research--what many people support. There's this thinking that humans are presently at this perfect balance and that everything would fall to pieces if we weren't. From the childbirth side of the scale it's true: Gotta be big enough! (But that's true for all animals that give birth through a bony birth canal.) However, it's not clear why it's true from the bipedalism side of the balanced equation.

And if turns out to be true that narrow hips do not contribute to male domination in sports (assuming male sport domination is on mother nature's radar), then tell me what is ideal about the male pelvis? Maybe you never thought about it that way, but by assuming that the female is compromised, the "ideal" status for males is implied.

Neither the existence of sexual dimorphism in pelvic dimensions, nor anything else that I can think of supports a tradition of placing suboptimal value on the female form. The female births the babies so if (iff) there’s an "ideal" it’s female. Selection maintains its adequacy for locomotion and for childbirth. If it didn’t, humans would have gone extinct.

To some that may still mean the female pelvis is compromised. To me, it's a multi-tasker and a good one.

Always look on the bright side of life. But up to a limit, please. 
A popular reaction to our paper is, But why the tight fit at birth? It's impossible to ignore! Why should childbirth be so difficult?

Answers of "because the baby is big and the birth canal is not" or "it's a coincidence that might mean nothing because clearly we overcome it just fine" or "I like to think it's just a coincidence that my finger fits perfectly into my nostril" ... these sorts of replies rarely appease a protester.

We already discussed adaptationism in this context, but it's fun to consider it further in more Panglossian terms.

When I imagine what it was like to first hear about the obstetrical dilemma back in the 1950s and '60s when it was first suggested, here's what I think my reaction would have been:

Hooray! The pains of childbirth (2) and the curse of helpless babies (3) are no longer Eve's fault but Evolution's! This is great. Eve was framed! Point for evolution AND point for feminism. Woot!

And what I've been yapping away about for four posts is not refuting that evolution's the process behind all this. Of course it is. Everything in biology is either evolution or it's magic.

However, evolution isn't always unicorns and double rainbows. Sometimes life just sucks.

For example, I'm going to die. I could look on the bright side and say that my decomposing carcass will nourish myriad life forms in the complex web of life that lives beyond my death in amazing and beautiful ways. And I often like to think about my molecules living on in a narwhal or a mango tree. But c'mon. Death sucks.

That's an extreme example, but I sense a similar need to make lemonade out of painful and dangerous labor and needy babies.  There must be some good reason...

As if the simple observation ...that childbirth works warts and all... is insufficient reason.

With everyone who was born to a mom who was born to a mom who was born to a mom, etc..., with all the billions of us here today, why are we refusing to accept human reproduction as adequate?

We are nothing less than a raging evolutionary success, just like nearly everything else that is alive right now.

Instead, the downsides to reproduction mean to some that it's deficient, leading them to seek reasons or evolutionary upsides: It's okay ladies, childbirth sucks because humans have such big wonderful brains! It's okay ladies, childbirth sucks so we can walk and run properly! It's okay ladies, babies are so needy and helpless so they get out into the environment where there's proper stimulation for learning and development!

But why do we need any other reason or evolutionary upside than the cute little bundle of joy?

http://blogs.smithsonianmag.com/hominids/2012/08/timing-of-childbirth-evolved-to-match-womens-energy-limits/

References/Notes
1. Simpson SW, Quade J, Levin NE, Butler R, Dupont-Nivet G, et al. (2008) A female Homo erectus pelvis from Gona, Ethiopia. Science 322:1089–1092.

2. The Book of Genesis (See where it all goes wrong after Eve first ate fruit with the serpent then ate it with Adam.)

3. Influential fourth century orthodox Christian, Augustine bishop of Hippo, called upon the fact that infants are born helpless to support his description of the sinful, suffering, terrifyingly vulnerable natural state of the human species. In Pagels, Elaine (1988) Adam, Eve and the Serpent. New York: Vintage.

***

And for your Friday happiness...this never fails to crack me up into all kinds of stitches... Jeff Tweedy reads "My Humps" (the song that my title of this post riffs on) by the Black-Eyed Peas.

How to apply an evolutionary hypothesis about gestation to your pregnancy

To get up to speed, click on Part 1 here and Part 2 here to learn about the paper I'm writing about below...Or read it for yourself in early view here at PNAS. 

There have been some very personal reactions to the press that came with our recent paper on the evolution of human gestation length.

And I don't mean this kind:

I mean the what about my short/ long/ weird pregnancy? kind.

Result of googling "weird pregnancy"

This research has always been wrapped up in questions about human variation and even draws upon observations of human variation in gestation length. So I'm not surprised it's causing people to reflect on their own experiences. And I'm also not totally surprised because I've been on the planet long enough to know that if you claim to know anything about pregnancy, you get all the stories.

But I didn't fully anticipate how strongly our work about humans as a species would be seen as work about "me." I guess we're only human.

So today's post is for all the people who read media about our paper and are dying to know what it's got to do with their own pregnancy.

Some things first.
1. I see the world through evolution goggles. Take that as close to literal as you can.

2. I have more scholarly experience with skeletonized (dead) and fossilized (extremely dead) humans than living ones.

3. I am not trained in medicine or health sciences.
4. I will not give medical advice.
5. I do not know what doctors are, or should be, telling pregnant women about eating and exercise.
6. It took me five years to write this paper from first notes to publication and I needed the help of brilliant experts to make it as strong as it is. I do not expect to fully appreciate its implications on the week it is published--not for human evolution, not for pregnant human mothers, not yet! If you have ideas... go on with your bad self and test them! I'll try to do the same.

Here we go, then...

How to apply an evolutionary hypothesis about gestation to your pregnancy

#1 thing to think about. 

Evolution is everything about you, but it is not all about you.

When reports of our research say "moms" we're not talking about you in particular. We're talking about "moms" in a general comparative evolutionary context, species-wide, primate-wide, mammal-wide.

#2 thing to think about. 

The EGG hypothesis explains species-level phenomena

Many evolutionary papers like ours are about understanding species level phenomena and comparing differences and similarities between species to better understand those phenomena, to explain whether patterns exist and, if they do, how or why.

So using the EGG hypothesis to explain why you gestated 9 days past your due date is a little bit like this: Try using the broad ecological and biological rules and patterns that explain variation in body size across mammals to explain why Fred the elephant is 9 cm taller than Frank the elephant. That's a challenge. That's what you're attempting to do if you read our paper (or reports on it) and think of yourself first rather than your species.

Here's another way to think about it. You might have seen our paper described as finding, "Metabolism, not the hips, limits gestation."  Metabolism might get you thinking of yourself but the hips hypothesis (obstetrical dilemma; OD) never did right? I could be so so wrong but nobody thinks that there's some way the fetus can sense when its head or shoulders are about to be too big to fit through the birth canal and then initiates labor so it can escape. Nobody thinks that the mom's body can detect when the baby is about to get too big to pass through her birth canal and initiates labor so it can escape. Nobody really thinks that these sorts of mechanisms exist in mothers do they? (It's possible but I don't know of any literature suggesting this.) So the hip constraint hypothesis (OD) was never about individuals, it's about our species over evolutionary history, with hips shaping our gestation length to be the right length for babies to escape in time. Generations over deep time... that's where your brain needs to be with this EGG idea too.

Sure, we need to consider individual human variation, like yours and mine. To formulate the EGG hypothesis we drew heavily upon Ellison's (2001; and in our paper) metabolic crossover (MC) hypothesis for the timing of human birth: Babies are born when they begin to starve in utero. This happens when the needs of the fetus surpass the mother’s ability to meet them or, in other words, cross over to become larger than what the mother can provide. Labor is then triggered and carried out by a complex biochemical process. Some of the evidence he provides includes:

Gestation length can be truncated according to metabolic parameters.

• Gestation is shorter in mothers with lower body fat composition and lower metabolic rates (Klein et al., 1989; Ellison, 2001)
• Mothers living at high altitude can give birth earlier than counterparts at low altitude (Lichty et al., 1957)

Gestation and fetal growth can be increased according to metabolic parameters.

• Fetal brain size pathology à longer gestation (Higgins, 1954)
• Increase maternal caloric intake à neonatal increase (Prentice et al., 1981)
• Increase maternal caloric intake à preterm births decrease (Prentice et al., 1981)

The MC is very much about individual within-species variation and it's possible that the MC explains all individual variation in gestation length among humans, however, that's uncertain right now. Since it's specific to the biochemical pathways of humans, the details of the MC don't apply to other species with different physiologies. However, the idea that human gestation is limited by mother's metabolism--the cornerstone of the MC-- is what EGG applies to human/hominin evolutionary history and to gestation in other primates and other mammals as well, since a mother's body size (a nice proxy for metabolism) predicts fetal size and gestation length across mammals.  This is really not news to a lot of researchers considering the body of research supporting it.

It's a useful method in evolutionary biology to look at variation within a species and use it to hypothesize why variation exists between species. That is what we have done with EGG. Mother's body sizes differ between species like say, humans and orangutans, and so do their metabolic traits! EGG suggests variation in metabolism between species explains variation in gestation length. It predicts that species do not exceed their species specific metabolic ceiling during pregnancy. It will be exciting to find out whether some species give birth well before they reach their metabolic capacity!

#3 thing to think about. 

Evolution is about common ancestry and change over time. "Ideals", "optimization," "standards," "greater value in this form, lesser value in that one"... these do not exist in nature except in our minds.

You worrying that you gestated too long or too little compared to the species average is a bit like you worrying that you're shorter or taller than average, have a larger or smaller head than average, have more saliva than average, or that you can't intentionally fart. Stop worrying about your normal variation. Variation exists because it works. There's safe wiggle room around most traits and sometimes there's even full-on spasmodic dancing room. We'd be extinct if there wasn't any room for variation in how to survive and reproduce. Celebrate your weirdness, your slightly long healthy gestation, your slightly short healthy gestation, your big healthy baby, your small healthy baby, your freckles, your asymmetrical face, your hairy knuckles, your lack of wisdom teeth, your pterodactyl toes. Who cares! If life's getting on with your weird ass, then you can certainly get on with life.

Further, it helps if you don't require EGG to be all about adaptation. It could be. But it's easier to think of it as just the way it is. Mothers can only gestate so long. Period. The mechanism that initiates labor based on those metabolic cues (MC)... totally adaptive! The process the EGG explains? Not really ... a limit's a limit! How could it surpass it? It would be physiologically  impossible. Adaptive ideas aren't necessary for EGG unless it's somehow adaptive to keep the fetus inside mother right up until that threshold. Which is possible. But it could also just be the only way to trigger labor. And so we're back to the EGG being just how it is.

***

So how should you apply this evolutionary hypothesis to your pregnancy?

It sheds light on why it's difficult to give birth. It sheds light on why babies seem so helpless compared to other primates.

But regarding your specific individual details that differ compared to other human mothers and their babies?  Please talk to your doctor who's your main brain on this. And read read read read read, if you're interested.

There are some pretty cool cultural and philosophical implications of our paper. I'll save those for tomorrow's post.


References
Ellison, P. 2001. On fertile ground: A natural history of human reproduction. Cambridge: Harvard University Press. [link to book on amazon]

Higgins LG. 1954. Prolonged pregnancy (partus serotinus). Lancet 2: 1154.

Klein, J, Stein Z and M Susser, (1989) Conception to Birth: Epidemiology of Prenatal Development. New York: Oxford University Press.

Lichty JA, Ting RY, Bruns PD, and E Dyar. 1957. Studies of babies born at high altitude. Part I. Relation of altitude to birth weight. American Journal of Diseases in Childhood 93: 666-669.

Prentice AM, Whitehead RG, Roberts SB, and AA Paul. 1981. Long-term energy balance in child-bearing Gambian women. American Journal of Clinical Nutrition 34: 2790-2799.

That [obstetrical dilemma] really tied the [human evolution] together. Part 2.

Update (Aug 30, 6:41 am): Paper's up. Here.

Update (Aug 28, 3:32 pm):  Jeepers, if I'd have known readership (err, clickership) was going to jump way above normal with this post, or that writers would lift quotes from here, I'm sure I would have crafted it better. 

Note: We're past our embargo, so I'm posting this now even though it doesn't appear that the article is posted on-line yet. I'll update this post and link to it once it is.

Some colleagues and I have a paper this week in early view at PNAS (1). I already told a big part of the story here.

In our paper, we show how weak--given current evidence--the popular obstetrical dilemma hypothesis (OD) is for explaining human gestation length. And, we offer up an alternative hypothesis as well.

It gets a lot bigger than this, doesn't it moms!  
(image: http://classes.biology.ucsd.edu)

The EGG hypothesis
What limits fetal growth during pregnancy? The OD says it's the pelvis--implying it's a unique constraint due to bipedalism. But the EGG hypothesis suggests that the primary constraint on fetal growth and gestation length is maternal metabolism (energetics, growth, gestation). Mothers give birth when they do because they cannot possibly give anymore energy into gestation and fetal growth. And when you look at the data available on pregnancy and lactation metabolism in humans... it shows that right around 9 months of gestation, mothers reach the energetic throughput ceiling for most humans.

Here's Herman's Figure 3 showing the EGG for humans, plotted with real metabolic data. Circles are the offspring, squares are the mother. Notice how fetal energy demands increase exponentially as the end of a normal human gestation period approaches. To keep it in any longer, mother would have to burst through her normal metabolic ceiling. Instead, she gives birth and remains in a safe and possible (!) metabolic zone.

The starred dot is a human infant at the developmental equivalency of a newborn chimpanzee. This is the thought experiment that Stephen Jay Gould famously wrote about. That's the age you'd have to birth a human baby to be like a newborn chimp, since we're born born more helpless than chimps. Keeping a fetus in this long--that is, adding 7 or more months to our gestation--would be physiologically impossible because it would require a mother to exceed 2.1x the basal metabolic rate, bursting through the ceiling for most humans. We actually gestate as long or maybe a little longer than you'd expect for a primate or a mammal, not shorter! So our relative helplessness at birth is indicating how much more neurological growth we have to achieve during our lives, after birth, than chimps and other relatives.

The EGG is a more general incarnation and a broader application of Peter Ellison's "metabolic crossover hypothesis" for the timing of human birth. The EGG branches out beyond our species, considering humans to operate within the physiological confines of other primates and mammals. But comparable data for other species, for testing the EGG, are not yet available to our knowledge.

Why do we grow babies that seem too big to fit through our birth canals? A strong hypothesis is that it's our diets that have radically changed compared to most of our evolutionary history. Many humans have constant and easy access to high calorie foods while pregnant and they can grow bigger babies over longer pregnancies. Very much related to this idea, check out Herman's recent NYT article about how our energy intake affects our health: "Debunking the Hunter-Gatherer Workout."

***

We named the hypothesis for ease of communication, not because we're eggomaniacs. We were tempted to call it HAM (humans are mammals) but felt that EGG better described the idea and was also adorable considering how babies are made.

HAM and EGG, or EGG and HAM, to me, is the ideal name but try saying that without going all Dr. Seuss on a wumbus full of thneed-suited who-scientists.

And that goes for here too. Your pop culture references best remain R-rated if you're to retain an ounce of R-word. And because it's just so enlightening, we'll continue to employ the very mature and refined Lebowski theme from Part 1 in our discussion here.

Call me Maude.

“The [species] abides.”
Part of the trouble I and others have with the obstetrical dilemma is this: We do just fine in the face of the tight fit at birth. Just because there's a tight fit, just because childbirth is terrifying, just because it's not an easy or enjoyable experience, that's not necessarily a "bad" thing evolutionarily. Clearly it's the opposite. It's a good thing. We're here to think about it! It can't possibly be "bad" if we keep having babies despite the hellishness of childbirth. This perspective was one of the contributions of "The obstetrical dilemma revisited" (2): Our behaviors, our aiding of women during childbirth, have probably reduced selection pressures against the tightness of fit, or other contributors to childbirth difficulty and danger. The species abides.

When you look at childbirth not as a biological failure, or as God's plague on lascivious women invited by Eve, but when you see it instead as a raging success, the obstetrical dilemma hypothesis is much easier to doubt.

The picture of evolutionary success. (source)

"Say what you like about the tenets of [Natural Selection], Dude, at least it's an ethos."

The widespread popularity of the OD may well be rooted in its adaptationist appeal, where nonoptimality (e.g. human altriciality, or helplessness and relative underdeveloped-ness at birth) is explained as a contribution to the best possible design of the whole (e.g. big brain and efficient bipedalism). Gould and Lewontin (3) famously criticized the “adaptationist programme” by cautioning that “organisms must be analyzed as integrated wholes” that are “constrained by phyletic heritage, pathways of development, and general architecture” and that “the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs.” They faulted this approach for failing to “consider alternatives to adaptive stories” and for its “reliance on plausibility alone as a criterion for accepting speculative tales.”

From this perspective, it's inappropriate to root the evolution of human altriciality in a compromise between adaptations for big brains and adaptations for bipedalism when there are most likely more basic, conserved, phyletic constraints on pathways of development and general architecture (e.g. gestation, pregnancy and fetal growth) at play.

“Yeah, well, that’s just like, uh, your opinion man.” 

Over the last five years as I’ve been thinking about this, specifically, I've had a teeny tiny bit of resentment creep up now and again towards the field that coaxed me into buying this OD as dogma. But I have nobody to blame but myself! A hypothesis is just that and why I just swallowed it whole without doubt is partly because it's a cool idea! And partly because an alternative idea just wasn't as well-known yet! Our paper is not attacking anyone, despite the guilt we've induced in folks who have been treating the OD as fact and teaching it to hordes of students for the last 50 years. To those researchers and teachers who came before us, we’re grateful! And to anyone who thinks of our paper as gotcha or an attempt at it: Please remember how science works and how knowledge accumulation works. That's all this is. It's just a little more hyped because it's about humans, not sea squirts.

The OD is not dead. It's just put in a less omnipotent place. The heaviest burdens should always be on supporting hypotheses for human exceptionalism; we should never default to them. Humans are animals/mammals/primates/hominoids and when we fail at that default view, that's when we can claim human exceptionalism.


References

1. Dunsworth HM, Warrener A, Deacon T, Ellison P, and H Pontzer (2012) Metabolic hypothesis for human altriciality. PNAS on-line early view.

2. Rosenberg K, Trevathan WR (2002) Birth, obstetrics, and human evolution. BJOG 109(11): 1199–1206.
3. Gould, SJ and RC Lewontin (1979) The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society of London, Series B 205(1161): 581-598.

Note
Please do look to the PNAS paper to read about the EGG or to see how we've exposed the challenges to testing the traditional OD. I did not write this post to stand for anyone's sole source. If you cannot access the paper once it's posted on-line, then please email me and I'll happily send the paper to you.

Another Update (Aug. 30, 6:41 am)

That [obstetrical dilemma] really tied the [human evolution] together. Part 1.

Some impressive colleagues and I* are about to have a paper published that pulls the rug out from under a classic paleoanthropological hypothesis/theory.

Scratch that. If we're gonna do this Lebowski theme right, I just have to say that our paper will have “micturated upon” an old beloved rug, I mean story.

Trust me. Our research did not come from a “shut the [bleep] up, Donny” dismissive sort of place nor did it come from a “the bums will always lose” holier-than-thou sort of place. Nothing like that.

It came from honest to goodness seeking to understand, man.

Here's the old beloved story:

The obstetrical dilemma (OD) hypothesis = Simultaneous selection for big-brained (or simply big) babies and bipedal locomotion caused a dilemma because while babies must be large, birth canals must remain small. The consequences of this dilemma, which are often called “solutions” and “tradeoffs,” include (1) difficult and dangerous childbirth with universal assistance due to the tight fit, (2) relatively underdeveloped, helpless, often termed “secondarily altricial” neonates compared to all other primates which are precocial, and (3) compromised or sub-optimal female locomotion, since (4) selection has favored sexual dimorphism in the human pelvis with females having not just relatively wider but absolutely wider dimensions of the birth canal.

Notice how--like the way that a nice oriental rug spiffs up a dumpy Los Angeles living room--the OD skillfully ties together many unique or fascinating phenomena in human evolution, such as human bipedalism, human encephalization, hellish human childbirth, helpless (i.e. hellish?) human babies, male-biased human athletic ability, and broad ladies' hips.

And we haven't proven this story wrong. But we have thrown some serious doubt on it, demonstrating how little of it holds up to current evidence.

“This is our concern, Dude.”

Way back in grad school, my wise advisor Alan Walker gave me a copy of Adolf Portmann's A Zoologist Looks at Humankind in which he argues against a pelvic constraint on human gestation and fetal growth (the OD). So that primed me to carry some doubt in this OD world.

Then later, in 2007, I was post-doc-ing with Nina Jablonski and immersed in the mammalian life history, energetic, and encephalization literature. It occurred to me that, Oh goodie! I’ll find out how selection could have shortened our gestation as we became encephalized but as selection also maintained our small birth canals for bipedalism.

And not only did I strike out all around. But, I mean, the mammalian life history literature looks as if there’s absolutely nothing constrained about human gestation length or the timing of birth. If anything it looks like we’re weirder in the other extreme… having slightly longer gestations than other primates and having relatively big babies. Leading up to birth we’re actually suped-up primates, not limited ones. That we're not particularly different in these terms, and definitely not limited, has all been known for decades. I was late to the party.

When you look at Bob Martin's work, and others' like it (below), you see that the size of the mammalian mother predicts the length of gestation and the size of the offspring.

Manger, PR. 2006. An examination of cetacean brain structure with a novel hypothesis correlating thermogensis to the evolution of a big brain. Biol. Rev. 81: 293–338. "Fig. 17. Allometric plot of the relationship between neonatal (Mbirth) and adult body mass (Mb) in three orders of eutherian mammals. The data used in this plot are derived from that given in Nowack (1999)."

These predictions hold even when you look across mammals that have single births or litters and note how this includes encephalized mammals, like whales, that don't even have bony birth canals!

Sacher GA, Staffeldt EF (1974) Relation of gestation time to brain weight for placental mammals: implications for the theory of vertebrate growth. Am Nat 18(963): 593-615....Using an equation that takes into account neonatal brain weight, litter size, and “brain size advancement” (neonatal brain weight ∕adult brain weight),they predicted the gestation length for species of animals with known gestation length. These few variables, which exclude any pelvic dimensions, were successful at predicting gestation length in the vast majority of species in their study, including humans and the cetaceans which lack constricting bony birth canals.

It seems so obvious that there's an energetic limit to what a pregnant mammal mother can do. And it seems so not obvious that humans are exceptional as the OD would have it. And this perspective was strengthened after I read Peter Ellison's book On Fertile Ground: A Natural History of Human Reproduction in my spare time in the field one summer, fitting in a few pages after each hot day of Proconsul hunting, after retiring each night in my canvas tent on Rusinga Island.


“This [hypothesis] will not stand, man.”

So with all this research out there showing how birth seems to be limited primarily by maternal metabolism, why this notion that we’re compromised by our pelves? Why this notion that we could or should keep babies in our wombs longer if it wasn't for bipedalism keeping our birth canals too small for gestating any longer, for growing bigger babies?

After all, there was Anna Warrener (now at Harvard) presenting her dissertation research at our annual conference showing how wide hips aren't so bad for locomotion. And she cited other papers with similar results!

So why hadn't the OD been reevaluated yet given all this stuff. I wasn’t sure. And to be honest, I thought it was so obvious, this disconnect, this weakness of the obstetrical dilemma hypothesis that after I completed my first draft of the manuscript in 2008, I decided not to do anything with it. It was so obvious, to me, that it became so absolutely ridiculously pointless to write about it. But once I mustered up the gumption to send the manuscript out to several close friends (like Ken and Anne!) for a read and none of those clever folks said it was ridiculous, that’s when I felt some encouragement. It wasn’t ridiculous, it made sense. I just happened to be the first person that my friends had known to put it together.

Or so I thought.

Here I was ready to put this idea out there and then one of my readers, Jeff Kurland, a brilliant and beloved professor from my grad school days at Penn State mentioned, “I'm pretty sure Terry Deacon presented this same thing at the AAPAs back in the 1980s or ‘90s.”

This is when your heart sinks because you just went to all that trouble only to find out that someone already beat you to it. Again, I went back to thinking that my ideas must be so obvious to everyone in the field now. But no searching came up with any Deacon pubs on the topic so I wrote to him directly and he confirmed that he hadn't published it, but he shared the manuscript that he'd stopped working on long ago. It fit with mine in many ways--this doubt of the certainty that the bipedal pelvis is limiting further gestation length and fetal growth--and since he'd presented it publicly, I asked if he'd like to be on the paper and he did. This is when your heart soars because someone so clever shares your thinking. This idea is not ridiculous! (Plus, even if he had published it already, my paper could have been a much updated contribution and still not pointless. Hopefully I would have understood that, or at least someone would have shaken some sense into me.)

“You’re out of your element, [Holly].”

Right around the same time as I was getting this nice feedback from colleagues and friends I spoke to another close colleague, Herman Pontzer, about it. He's known as the "energetics guy" among other things so I figured he was the perfect litmus. And it seemed fairly straight forward to him. Again, it's not ridiculous. Hooray! I wasn't crazy! Plus now I had this fully capable human being on board, ready to replace my stolen figures from other pubs, to make similar points but with updated data. And, even better, he could test this hypothesis about maternal energetics by plotting out the data from various data sources. All was falling into place.

"This is what happens when you [birth] a [baby] in the [pelvis]!" ... “A world of pain.”



As we were putting our story together, I joked to a brand new mother, "You know, there's no obstetrical dilemma." And got a sharp-tongued, Oh yes there is, honey. I just went through labor. Hell yes there is!

In future such discussions, I was always sure to add, "...it's all energetics. A mom gives birth when she does because she can't possibly give any more energy into growing that fetus." And some moms who hear that are like, Duh! I could have told you that! 

You cannot win. At least I could not. But it was still encouraging.

By this point, we (Anna, Herman, Terry and I) had joined forces with Peter Ellison and we submitted our paper to a major journal for review. And I'll be back with a little digest of that paper and some thoughts on it a bit later...

Update, Sept. 1, 2012: Here's the next post. 

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*Feynman's "half-advanced and half-retarded potentials" describes us nicely, with me as the latter.