I read a rather interesting paper recently looking at the development of the human female pelvis, from late fetal stages until late adulthood. (You can find the full text for free here). The paper describes itself as challenging the “obstetrical dilemma hypothesis”: this is the idea that there are conflicting demands on the human female pelvis: to efficiently walk on two legs, a narrow pelvis is better, but to have large-brained babies, and to give birth safely to them, a wide pelvis is better. What I also find interesting, however, is that it takes the subconscious assumption that a large, bony structure, such as the human pelvis, is relatively fixed in proportion once you reach adulthood: turns out that, for women at least, it’s not.

It’s fairly widely accepted that the female pelvis has evolved its shape as a compromise to the two conflicting selective pressures of efficient locomotion and birthing babies with large heads: the pelvis has become wider at the expense of walking (or running) efficiency, but human babies are also born at a comparatively early developmental stage: they are very immature and helpless compared to other apes (or, indeed, other mammals). Males, by contrast, don’t have this problem, and so they have narrow pelvises. Take a look at this rough diagram showing just how constrained the pelvic opening is in modern humans versus a chimpanzee, or even an early hominin, Austrolopithecus:

pelvis comparison
ArchaeoMouse (own work): https://commons.wikimedia.org/w/index.php?curid=44884807

This incredibly tight fit can be a big problem: there’s a high prevalence of obstructed labour and, of course, the maternal (and infant) mortality and morbidity rate without modern medical assistance is high. Other data and ideas have also challenged the OD hypothesis: The energetics of gestation and growth (EGG) hypothesis proposes that the timing of birth is constrained by the limited metabolic output of the mother rather than by spatial limitations of her pelvis: essentially, there comes a point at which the energy demands of the fetus (heightened by its large brain mass) exceed the maternal ability to meet them, and this occurs at roughly 9 months, when birth occurs. (The relevant paper, also quite recent is here, but there’s a nice Guardian article here that’s far easier for the non-specialist). Most strikingly, experimental data indicate that a wide pelvis does not reduce bipedal locomotor efficiency. Wait, what was that? The whole idea that the human female pelvis is some sort of awkward compromise between walking and birthing might not actually not true? So why, millions of women are probably asking themselves, did I have such a hard labour? And, I was asking myself, was this one of those “hypotheses” that was born out of “everyone knows that, it’s obvious…” assumptions without really checking the data properly?

Well, obviously there’s other issues in there, but the high prevalence of obstructed labour (yes, it basically means the baby got a bit stuck, yes it’s every bit as un-fun as it sounds) still demands an explanation. After all, if it doesn’t really affect the efficiency of locomotion, why don’t women have wider pelvises? The paper’s introduction discusses a few possibilities here, but they’re pretty hand-wavy, and what it boils down to is: well, actually, we don’t know. Nobody has really looked hard enough before, because, well, they all thought they knew the answer.

The authors of this current paper proposed an alternative hypothesis: the developmental obstetric dilemma (DOD) hypothesis, which posits that the size and shape of the pelvis changes, reflecting changing obstetric needs during a female’s lifetime. Specifically, this hypothesis predicts that the female pelvis would start to diverge in structure from the male one after puberty, and be at its optimum shape for giving birth at a time that coincides with peak female fertility (25-30 years old); that after the menopause, it would revert to a more narrow morphology; and that the male pelvis won’t show these changes over time. Importantly, all these hypothese are testable.

The authors used computerised tomography to look at the 3D structure of various pelvises, from males and females of different ages, spanning the late fetal stages to late adulthood. Looking at a number of anatomical markers, they plotted variations in size and shape. I won’t reproduce the graphs or the jargon-heavy anatomical descriptions here, but the upshot is that they confirmed their hypothesis:

With the onset of puberty, the female developmental trajectory diverges substantially from the childhood trajectory, whereas the male trajectory essentially continues its earlier course. As a result, the female pelvis attains its obstetrically most favorable morphology around the age of 25–30 y, i.e., at the age of highest fertility. Furthermore, pelves in postmenopausal women assume a developmental mode that is largely similar to that of males, with the effect that the birth canal becomes constricted.

pelvis
Fig.3. Anterior, superior, and lateral views showing male and female patterns of pelvic shape change from ~15 y (transparent) to ~25 y (solid) (A), and from ~40 y (solid) to ~80 y (transparent) (B). Taken from Huseynev et al, 2016, PNAS.

What causes these shape changes? Well, probably oestrogen, feeding into steroid and growth hormones, but it’s not entirely known. It does suggest that there is some advantage (possibly locomotor) to a narrower pelvis, which may be to do with locomotion, although I remain unconvinced. So why the high prevalence of obstructed labour? The authors suggest that ecological and nutritional factors affecting oestrogen levels, in particular, may be a factor. The EEG hypothesis would suggest that your hips got “wide enough” (and that’s all evolution needs) for a baby that would be born after 9 months, and not any wider. However, under rapidly changing environmental conditions (by which they mean something like “significant change in nutrition” rather than “sudden ice age”) there may what they call a “fetopelvic mismatch”. I can easily see how if your diet as a child is significantly different from your diet as an adult, even if it’s only just a more/less plentiful food supply, then you might end up with a pelvis that doesn’t quite match up to the fetal growth when you’re pregnant. It’s also worth noting that the increasingly early age of menarche (when girls/women start menstruating) in developed countries is linked to increased nutrition levels, particularly fat, which feeds into the steroid hormone pathway and affects oestrogen levels.

I can see that if the Daily Mail got hold of this story it would no doubt twist this into some tedious sexist rant about older mothers, and how women shouldn’t have children past 40 because their pelvises aren’t up to the job, and how unnatural this is when they do. Of course the reality is that there are a huge variety of factors affecting whether a woman will have a safe and “easy” childbirth or not, of which this is probably not even one of the major ones. Leaving aside the fact that women have been having children in their forties for centuries, it just was usually their fifth or sixth child, what I find most interesting is that peak fertility doesn’t start until about 25 years of age. (Also, interestingly, when your brain is actually fully mature). Why, if your pelvis doesn’t reach it’s “best” shape for childbirth until you’re about 25, are most women fertile significantly earlier? Well, again, there’s all sorts of evolutionary trade-offs going on here. For one thing, going back to our ancestral hunter-gatherer lifestyles, you’re more likely to die before you’re 25 than before you’re 18, so there will be pressure to have babies younger. Also, of course, you can fit more into that fertile window if you start younger – particularly as there will be a high mortality rate. There’s a bit of confusion in this though: the transition to a settled, agricultural life resulted in much higher fertility rates for women. However, generally, there was a lot more death and ill-health, both for the children and the women, and it remains the case that childbirth, even with modern medicine, is a dangerous business. If your mother dies giving birth to her eighth child, then her first seven children, particularly the younger ones, are going to see their own chance of survival dramatically reduce.

Oh, and in case anyone asks that other age-old conundrum: why do women become postmenopausal in the first place (with their presumably narrowed pelvises?). Wouldn’t it make sense to remain fertile, as men (mostly) do? Well, no, as you’ll probably just die giving birth when you’re physically not up to it. There is likely a point where it makes more sense to invest your energies in the children you do have, rather than having more.

References

Huseynev, et al, (2016): Developmental evidence for obstetric adaptation of the human female pelvis. PNAS, vol. 113, no.19, 5227-5232.

Dunsworth, et al, (2015): Metabolic hypothesis for human altriciality. PNAS, vol 109. no 38, 15212-15216.

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