Why Humans Give Birth to Helpless Babies


Human babies enter the world utterly dependent on caregivers to tend to their every need. Although newborns of other primate species rely on caregivers, too, human infants are especially helpless because their brains are comparatively underdeveloped. Indeed, by one estimation a human fetus would have to undergo a gestation period of 18 to 21 months instead of the usual nine to be born at a neurological and cognitive development stage comparable to that of a chimpanzee newborn. Anthropologists have long thought that the size of the pelvis has limited human gestation length. New research may challenge that view.

The traditional explanation for our nine-month gestation period and helpless newborns is that natural selection favored childbirth at an earlier stage of fetal development to accommodate selection for both large brain size and upright locomotion—defining characteristics of the human lineage. In this view, adaptations to bipedalism restricted the width of the birth canal and, hence, the size of the baby that can pass through it. Human babies are thus born when their brains are less than 30 percent of adult brain size so that they can fit through the narrow passageway. They then continue development outside of the womb, with brain size nearly doubling in the first year.

But when Holly M. Dunsworth of the University of Rhode Island and her colleagues tested this so-called obstetrical dilemma hypothesis, their findings did not match its predictions. For example, the hypothesis predicts that because the female pelvis is broader than the male pelvis, walking and running should be more energetically demanding for women than for men. Yet most studies of the energetics and mechanics of locomotion in women and men found no such penalties for having a wider pelvis, the researchers report.

Furthermore, the team asserts, to accommodate an infant at a chimplike stage of brain development—that is, a brain that is 40 percent of adult brain size, or 640 cubic centimeters—the pelvic inlet (the top of the birth canal, which is the narrowest part) would only have to expand by three centimeters on average. Some women today have pelvic inlets that wide, and those larger dimensions have no measurable effect on locomotor cost. The researchers argue that instead of fetal brain expansion being constrained by the dimensions of the pelvis, the dimensions of the human pelvis have evolved to accommodate babies, and some other factor has kept newborn size in check.

Written By: Kate Wong
continue to source article at blogs.scientificamerican.com


  1. Have you see the size of a full term belly?  The woman could barely walk at 9 months.   That being said, unless the fetal body remains the same size for a  few extra months (allowing for brain development) it would be prohibitive to go over the typical gestational period.  Not to mention placental life is also on its own timeframe; at 9 months it’s close to ‘dying’ out and if a woman is 2 weeks after expected delivery date she is induced primarily for this reason.

    Essentially, we’re programmed to give birth at 9 months with the low brain development that you mention.  It just evolved this way; otherwise we’d just drop out and go surf the internet immediately.

  2. What about the energy cost to the woman of supporting a fetus for more than nine months?  It takes an incredible amount of energy in the form of nutrients and increased blood flow (new blood vessels and blood volume, increased cardiac output) to support a fetus and the placenta; this could have been limited by availability of nutritional resources.  As pbsoko commented, at nine months, a woman’s mobility can be severely limited by both the size of her belly, which shifts her center of gravity and impairs her balance, and the hormonal changes that loosen the pelvic joints to facilitate birth.  The heart expands and assumes a more horizontal position in the chest, the stomach, intestines and bladder are compressed, and lung capacity is reduced.  These changes, if prolonged, could have been a threat to survival in our early ancestors, so I imagine that birth had to occur as soon as the fetus’ lungs were mature enough to support it outside the womb, otherwise both mother and fetus might die. 
     Furthermore, it isn’t just the “locomotor cost” of giving birth to larger infants that has to be considered.  There’s the energy cost of labor, the pain, and the blood loss involved at birth.  Anecdotally, my daughter was more than 2 weeks overdue, weighed 11 pounds, and was 23 inches long at birth.  I nearly died (my OB was on vacation, and his sub was reluctant to induce me for some reason; even though they knew the baby was large, they did not perform a C-section) in labor and so did my daughter; my pelvis was separated at the symphysis, and her collarbones were broken.   The placenta had to be removed surgically, I hemorrhaged massively, and only blood transfusions and quick surgery saved my life (the legal repercussions are another story).  My point is that increasing the size of the infant’s brain isn’t just a simple matter of having an adequate pelvic inlet – there are many more physiological problems involved.

  3. Hum, that was one of those evolutionary quirks that I thought was pretty much understood. Very interesting to see it being challenged like this. Sue Blue also wrote a great comment on what other factors could have lead to the gestation length of human fetuses being so relatively short. Just thinking though, if human’s walked on all fours, would it be easier for a woman to carry a bigger fetus (more stable center of gravity, less compressed internal organs)? There would still be the issue of the bigger fetus draining the resources of the woman’s body.

  4. No penalties for having a wider pelvis in the mechanics of locomotion? That is questionable.

    It is said that one reason women’s tennis is less spectacular than men’s is that women are less efficient than men at moving sideways suddenly. Perhaps this is a theory developed to bolster sexist prejudice, rather than a scientific observation, but as a fairly casual tennis watcher I think it is plausible.

    Just watch women walking, from behind (all in the cause of science, you understand). Look at all the complex wiggles that are considered sexy by the male. Sexy, yes, but certainly not efficient. 

    Then watch the arms. A walking man swings his arms to a fairly small extent, parallel to his path. A woman’s arms swing outwards. Try passing a woman on a crowded path.  Watch women track athletes, whose wildly flailing arms often inconvenience the athlete in the next lane.

  5. Once an infant is helpless past a certain point, it may not really matter just how helpless. If three more months in the womb led to a newborn with no more survival ability than a 3-month old, that may not be much help. I’d be curious, though let’s not test it, who can better protect her offspring (or soon-to-spring-off)–a woman 12 months pregnant, or a woman who had given birth three months earlier. 

    Can a social group assist a baby more once it’s born than they can help protect a pregnant woman? Whether or not they can, do they? Empathy might be stronger for a baby than for an adult.

    A shorter gestation period also means a woman could bear more children; with shorter lifespans that may have been especially relevant. 

    I’m certainly curious why humans are born so immature, but I’m more mystified as to why childbirth is so dangerous to women. What other species face such risks? And even if slightly wider hips led to a slightly slower speed, how often did people run from lions and die by juuust that much, compared to dying in childbirth? Are wider hips just a difficult feature to evolve, and we would have them if we could have them?

  6. Okay, no expert here, but…

    Isn’t the increased danger to female humans during childbirth (in comparison to our nearest relatives) the result of larger brains in that larger brains gave us cooperation and culture – and cooperation and culture will, to a large extent, bypass and inhibit natural selection in certain aspects of our development but not others.

    So, for example natural selection continues to select for say greater intelligence/larger brains, but is inhibited to select for easier childbirth by the very culture that the larger brain provides, ie, care and assistance in childbirth.

    There must have been a tipping point in our history where it was touch and go for us as an upright species with a small brain. Remember, we were bipedal long before gaining larger brains. Posture giving us the hands that gave us the brain.

    It’s an interesting thought that had our cooperation/culture not been sufficiently developed to assist in childbirth at this crucial point then natural selection could have selected for easier births, wider pelvis, knuckle-walking and smaller brains (or brains that remained a similar size over the equivalent evolutionary period) with evolutionary success provided by easier, less dangerous reproduction.

    This could have easily occured due to environmental change causing specialisation from ground locomotion to tree or forest dwelling – removing the hands that gave us the brain.

    I’ve always been drawn to the idea that knuckle walking as seen in chimps, gorillas etc’, was an evolutionary development from bipedalism.

    It’s an equally interesting thought, is it not, that a form of universal healthcare may have been the inciting trait that made us the apex predator we are.

    The ‘biblical curse’ of the pain of child bearing – a product of our very humanity.

    Someone ought to tell the Tories.


  7. For every complex problem there is an answer that is clear, simple, and wrong.

    The definition of the problem is where the error lies. Growing a new humans isn’t fundamentally different compared to any other mammal species, including marsupials. Humans happen to be more marsupial-like than other mammals. We don’t have natural pouches but we make the necessary pouches out of other animals anyway. We know them as prams and slings.

    It doesn’t matter whether a foetus is carried internal or external, so the exact timing of birth is irrelevant. Whether birth occurs at any particular size of the foetus is a matter of convenience for the relevant species’ female lifestyle. Some babies are easier to carry around internally versus externally, at various stages of development. For a highly mobile species like humans, internal transport of relatively large foetuses implies massive volumes of placental fluid and inefficient placental tissue. All this stuff is quite heavy. Selection pressure may have convinced our species that it’s best to ditch it and focus on carry the foetus externally while accumulating internal body fat to convert to milk, depending on the source of body fat. That makes it easier to follow the reindeer, woolly mammoth etc. (Can leave the kids for a day or so with the old folks and tend to business.) The typical point of externalising offspring for any species, whether at the egg stage like birds and reptiles, early foetus like marsupials, later foetus like humans, or nearly mature offspring like horses,  fully alert and ready for fight/flight responses, is an accident of evolution.

    So the answer to the problem of what drives the timing of birth relative to the capability of the intended offspring is kind of missing the point.

    I vaguely recall hearing that North American native Indians didn’t even both naming children until they’d attained some level of childhood development. You could take things further and redefined foetal development relevant to the capabilities expected of typical mature adult humans. I’d say that being born at age approx. 9 months is not the defining moment. Perhaps reaching the age of 35, while in gainful employment, no longer living at home with mum and dad, and without attracting a criminal record, might be a better arbitrary definition of the cessation of foetal development in humans.

    Perhaps the real question is why human children remain dependent and immature until around their mid 40’s, an age when they are then at greater risk of becoming highly cynical?

  8. Pete H,

    Like your post, but surely the question is ‘Why Humans Give Birth  to Helpless Babies?’ and what can this tell us about us?

    Growing new humans ‘does’ seem to be fundamentally different to how our closest relatives grow themselves.

    We may happen to grow humans in a more ‘marsupial-like’ way but we are not marsupials, are we.

    They, and their closest relatives originated in south Asia (I think?) whereas we, and ours, originated in Africa.

    So, answering the question, ‘Why Humans Give Birth to Helpless Babies’ may tell us a great deal about our evolutionary path, and that of our closest relatives.

    Indeed it may tell us that whilst other creatures externalising the development of offspring was an ‘accident of evolution’ – ours was not.


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