First things first. Let’s get you up to speed on the gestation research that fed the media that fed the pregnancy questions that sparked this post.

Some impressive colleagues and I just published a paper (1) that pulls the rug out from under a classic anthropological hypothesis that suggests the bipedal-adapted human pelvis constrains human gestation and fetal growth.

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 (a) difficult and dangerous childbirth with universal assistance due to the tight fit, (b) relatively underdeveloped, helpless, often termed “secondarily altricial” neonates compared to all other primates which are precocial, and (c) compromised or sub-optimal female locomotion, since (d) 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 the OD skillfully ties together many unique or fascinating phenomena in human evolution such as human bipedalism, human encephalization, hellish human childbirth, helpless human babies, male-biased human athletic ability, and broad ladies' hips. And we haven't proven this popular story wrong. But our paper throws some serious doubt on it, demonstrating how little of it holds up to current evidence.

For starters, in primate and mammalian comparative contexts, human gestation and fetal growth are not constrained. If anything it looks like we’re weirder in the other extreme, having slightly longer gestations than other primates and having relatively big babies. That we're not particularly different in these terms, and definitely not limited, has all been known for decades, as has the understanding that the size of the mammalian mother (a useful proxy for metabolism) predicts the length of gestation and the size of the offspring.

With all this research out there showing how gestation 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? It’s unclear especially given how there is little evidence that wider hips are bad for bipedal walking and running or that slightly wider ones that would make childbirth easier or that would accommodate a more developed neonate would be problematic.

So as an alternative to this weakened hypothesis that the human pelvis constrains gestation length and fetal growth we offered up a new one born of the metabolic observations described above.

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).

The EGG hypothesizes that mothers give birth when they do because they cannot possibly give any more energy into gestation and fetal growth. And when you look at the data available on pregnancy and lactation metabolism in our species, it suggests that right around 9 months of gestation, mothers reach the metabolic ceiling for most humans.

Here's Herman’s Figure 3 from our paper, 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 feasible metabolic zone.

The starred dot is a human infant at the developmental equivalency of a newborn chimpanzee. This is the thought experiment that Adolf Portmann (2) and Stephen Jay Gould (3) famously wrote about. That's the age you'd have to birth a human baby to be as advanced as a newborn chimp, since we're 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 and sustain 2.1x the basal metabolic rate (BMR)—the ceiling for most humans. Our relative helplessness at birth is indicating how much more neurological growth we have to achieve during our lives, after we’re born, than do chimps and other relatives with their smaller brains than ours.

The EGG is a more general incarnation and a broader application of Peter Ellison's "metabolic crossover hypothesis" for the timing of human birth (4). 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. This is one of the infrequent times you’ll see a human model that’s hypothesized to work for other species rather than the other way around!

We named the hypothesis, EGG, 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.

Part of what has caused many of us to struggle with the OD is that humans 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. 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 widespread popularity of the OD may 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 (5) 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 the adaptationist 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, the EGG is preferred over the OD. Rather than rooting the evolution of human altriciality in a compromise between adaptations for big brains and adaptations for bipedalism, instead it’s explained by more basic, conserved, phyletic constraints on pathways of development and general architecture at play (e.g. gestation, pregnancy and fetal growth).

The OD is not killed by the EGG. It's just put in a less omnipotent place. The heaviest burdens should always be on supporting hypotheses for human exceptionalism; we should not default to them. Humans are animals/ mammals/ primates/ hominoids and when we fail at supporting our default view, that's when we can claim human exceptionalism.


There have been some very personal reactions to the press that came with our recent paper on the evolution of human gestation length. I don’t mean the what do you mean we evolved? kind. I mean the what about my short/ long/ weird pregnancy? kind.

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.

The following 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 (1) 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 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 at which point it 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 at which point she initiates labor so it can escape. Nobody really thinks that these sorts of detectors and mechanisms exist in fetuses or 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 Peter Ellison's metabolic crossover (MC) hypothesis for the timing of human birth (1, 4): 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 (4,6)
  • Mothers living at high altitude can give birth earlier than counterparts at low altitude (7)

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

  • Fetal brain size pathology à longer gestation (8)
  • Increase maternal caloric intake à neonatal increase (9)
  • Increase maternal caloric intake à preterm births decrease (9)

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. (Again, this is not news considering the body of existing research.)

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!

Why do we grow babies that seem too big to fit through our birth canals? It’s possible that it’s mainly a recent phenomenon and a strong hypothesis is that 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. There are probably genetic, epigenetic, and environmental affects on our modern metabolisms as well. Very much related to these questions is Herman’s recent article in the New York Times: "Debunking the Hunter-Gatherer Workout."

#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 could also be 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! It would be physiologically impossible to exceed it. 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 the way it is.


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

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

But regarding your specific, individual details about gestation length and neonatal size 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.


    1. Dunsworth H., Anna Warrener, Terrence Deacon, Peter Ellison, and Herman Pontzer (2012) Metabolic hypothesis for human altriciality. PNAS on-line early view.
    2. Portmann A (1969) Biologische Fragmente zu einer Lehre vom Menschen [A Zoologist Looks at Humankind] (Schwabe, Basel, Germany); trans Schaefer J (1990) (Columbia University Press, New York). German.
    3. Gould SJ (1977) Ontogeny and Phylogeny (Harvard Univ Press, Cambridge, MA).
    4. Ellison P (2001) On fertile ground: A natural history of human reproduction. Cambridge: Harvard University Press. [link to book]
    5. 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.
    6. Klein, J, Stein Z, Susser M (1989) Conception to Birth: Epidemiology of Prenatal Development. New York: Oxford University Press.
    7. Lichty JA, Ting RY, Bruns PD, Dyar E (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.
    8. Higgins LG (1954) Prolonged pregnancy (partus serotinus). Lancet 2: 1154.
    9. Prentice AM, Whitehead RG, Roberts SB, Paul AA (1981) Long-term energy balance in child-bearing Gambian women. American Journal of Clinical Nutrition 34: 2790-2799.