The phrase “childbearing hips”, besides being extremely cringey in any context, is a misnomer. It’s a byproduct of the kind of thinking that went into the Obstetrical Dilemma: women have to put up with less efficient walking in order to just barely squeeze out an underdeveloped baby. It’s a hypothesis that was widely assumed to be true– and unfortunately is still perpetuated by people who should know better. As it turns out, pelvic width has nothing to do with bipedal efficiency, nor is it a constraint for fetal head growth or childbirth. In fact, natural selection favored bigger babies right along with narrower bipedal pelvises millions of years before the first obstetrician was kicking his mother’s ribs.
A short note before I dive in: this entire discussion is focused on species level traits, sometimes dipping into population-level traits for a particular culture. This means that individual traits aren’t the focus, though they are very important to consider. An individual may have an individual trait that means they truly cannot give birth (or be born) vaginally due to pelvic constraints. For example, uncontrolled gestational diabetes, bone deformations or injuries in mother or fetus, conjoined twins, etc. For these and many other evidence-based, medically-indicated reasons, unrelated to species-wide fetopelvic morphology, c-sections can be life-saving. However, being a human and pregnant is not in itself a medical indication for medical interventions in labor, let alone a surgical delivery.
Bipedal Efficiency vs Birth
The foundation of the Obstetrical Dilemma (OD) —aside from a delightful combination of sexism and racism— is that there is a struggle for females between being able to walk efficiently and still be able to give birth to big-headed offspring. It is based in the idealization of the male form. The assumption that men’s narrower pelvises made them more efficient was repeated in the scientific literature as a basis for the OD for almost 70 years Without. Ever. Being. Tested. /Quelle Suprise!/
“Despite the wide acceptance of the obstetrical dilemma model, the effect of increased pelvic width on locomotor costs has never been directly addressed.” Warrener, et al. 2015Warrener, et al., 2015
That is, until Warrener and her colleagues set to the task of testing whether hip morphology affected bipedal efficiency, fully expecting to support the OD hypothesis. Yet, even in the face of their own data contradicting the untested OD hypothesis, they try desperately to cling to the myth that fetal heads are regularly too big to fit through the birth canal, “[…] pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running. Since a wider birth canal does not increase a woman’s locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal” (Warrener, et al. 2015).
It seems strange that as researchers finally testing a long held assumption, narrow hips = bipedal efficiency, only to discover it was wrong should then blindly accept another myth: that neonates too large to fit through the birth canal are prevalent. But all of us, including scientists, have our cultural lenses to deal with. In reality, fetopelvic disproportion is an extremely rare occurrence owing to disease or deformation (Dunsworth, 2016) and the source of many birth complications are the result of unnecessary, but routine, interventions during the labor process resulting in a surgical delivery.
An Appeal to Tradition
“the significance of human childbirth as an important source of selection on human pelvic morphology has been recognized for many years”Trevathan and Rosenberg, 2000
Throughout my research for this series, I found a lot of the proponents of the Obstetrical Dilemma hypothesis resorting to fallacious arguments to support its validity, as though wide-spread acceptance of an idea for a long time makes a fact. It was surprising to find professional scholars making the assumption that OD was true because other people said it was true “for many years” without checking the data (or noting the lack thereof).
But back to the point: We now understand that pelvic width has nothing to do with one’s bipedal efficiency. So, why don’t we have much much wider pelvises to make childbirth easier? so that we can gestate bigger-brained and more fully developed offspring? I mean, if the whole human birthing situation is as dire as proponents of the OD hypothesis have made it out to be, clearly, there should be some selection pressure there to evolve super wide “Childbearing Hips”?
The Obstetrical Dilemma hypothesis came about when hospital birth was being established as the norm in the 1930’s and ’40’s. Anthropologists at the time believed that modern (white) humans were considered a very recent exceptional species with unique problems giving birth due to bipedalism and large heads requiring obstetrical technology and technique to manage.
But anthropologists have learned a lot since then (at least, some of us have), for example, we have learned that bipedalism and the trend for big babies goes back far beyond our species, over four million years, at least to the species Australopithecus afarensis (aka “Lucy”). Something interesting happened with her species: just as narrower bipedal hips became en vogue the IMMR (infant mother: mass ratio) shot up from 2-3% in Lucy’s predecessors to 5-6% (DeSilva, 2011). In other words, natural selection favored bigger babies right along with narrower bipedal pelvises. Even today, our cousins in the Great Ape family have newborns with a 2-3% IMMR while we have newborns at the 5-6% IMMR.
Curiouser and Curiouser.
So what does this all mean regard to the Obstetrical Dilemma Hypothesis? I think that it means that pelvic width has little to do with birth and everything to do with pregnancy. The pelvis is a platform for the legs to attach to and to support the squishy organs of our torsos, for bipedal females one particular squishy organ may be occupied by an awkwardly large and growing fetus that needs a wide and expanding support structure. The body (the BOOTY) knows what it’s about– how many pregnant women have noticed their butt mirroring their growing baby bump?– it’s all about keeping balance.
Whitcome (2007) described Lucy’s experience of pregnancy, of “carrying a heavy fetal load” as causing similar fatigue and lower back pain as a modern human feels during later pregnancy. It was the fetal load in early bipedalism that selected for lordosis, or the curvation of the spine, which becomes greater during pregnancy to help maintain the maternal center of mass (COM) while walking.
Through hominin evolution, at least since Lucy’s time, narrower hips have been selected for even as fetal head size increased. That means that something is beneficial enough about our narrow pelvis together with our big-headed babies that they helped our evolutionary ancestors to survive and pass on their narrow-hipped, big-headed traits to all 7.7 billion of us living today— or that it’s totally unrelated and we’re all stupid monkeys in shoes.
Proponents of OD hypothesis love their diagrams: a one-dimensional solid pelvis with a solid circle to represent the fetal skull. The dimensions between the human pelvis and human fetal skull vary between them– it’s often the artist’s views on childbirth that affect the fit rather than any hard data. In line with the perception of the artist, the diagrams fail to show the movement of birth; the expansion of the pelvic joints, the pelvic muscles, the flexibility of the fetal skull and body, or the motion the human fetus takes around the pelvic inlet and outlet, which are always wider than shown in the diagrams.
Even at the current average human pelvic width, there is still plenty of room for fetal heads to grow. Both Epstien (1973) and Dunsworth (2012) found that to give birth much larger fetal heads (as in, toddler sized) the increase to the female pelvis would need be only 3-4 cm which is well within normal size differences between human females alive today.
“Thus, to accommodate a brain four times larger than that currently found in newborns would occasion an increase in brain dimensions of only the cube root of four which is about 1.6. […] A brain 1.6 times larger would be about 11 cm in diameter. The thickness of the skull bones need no increase at all. Thus, women have to be only about 4 cm wider in the hips on the average than now. As this 4 cm increase undoubtedly lie within the range of variation found among females today, there would be little noticeable difference if the human baby were born with about four times as much brain as now.” Epstein, 1973
“Modeling the neonatal head as a sphere and accounting for soft tissue, the diameter of a human neonatal cranium would be approximately 3 cm larger at 16 months, which is when the minimum hypothetical development equivalent to a chimpanzee newborn is reached. In order to deliver this larger neonate, the maternal pelvis would need to accommodate an additional 3 cm within the dimensions of the birth canal. The most constricted mediolateral dimension of the birth canal, the bispinous diameter, already varies by 3 cm among human female populations, without any obvious or systemic impact on locomotion. This run contrary to the expectations of the OD that ‘women couldn’t walk’ if the birth canal were widened to accommodate a more developed neonate.” Dunsworth, 2016
Molded neonatal heads are often pointed to as evidence that the birth canal is too small for the baby’s head, but I propose that the fetal head molds on its way out because it can, not because it has too. Fetal skulls are not made of solid bone but rather unconnected plates of bone to allow for the rapid brain growth in the years after birth, they finally fuse when the person has reached adulthood. Having plates of bone rather than a solid skull was not an adaptation intended to ease birth but to allow the brain to grow. Besides, where the brain is squishy the shoulders are fairly rigid and they need to pass through the birth canal too.
On every point, the Obstetrical Dilemma is incorrect. Female bodies are no more or less efficient than a male body in walking or running and pelvic width is not a constraint on the fetal size or on birth at the species level. So, whenever you hear someone referencing women’s pelvic width with regards to their ability to run, walk- or give birth– let them know it’s 100% bullsh*t. Or just send them a link to this post. /wink/
In the next part, I’ll be covering why newborn humans are so useless and you guessed it– nothing whatsoever to do with pelvic morphology. It’s all about social learning and oxygen. Subscribe for updates.
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Sources (for the entire series):
Belaunde, Luisa Elvira. 2000. “Women’s Strength: Unassisted Birth Among the Piro of Amazonian Peru.” Journal of the Anthropological Society of Oxford 31:1, 31-43.
Davis-Floyd, Robbie E. 2003. Birth as an American Rite of Passage. 2nd edition. Berkeley: University of California Press.
Dent, Albert W. 1949. “Hospital Services and Facilities Available to Negroes in the United States.” The Journal of Negro Education: The Health Status and Health Education of Negroes in the United States 18:3, 326-332
DeSilva, Jeremy M. 2011. “A shift towards birthing relatively large infants early in human evolution.” PNAS 108:3, 1022-1027.
DeSilva, Jeremy M. 2016. “Brains, Birth, Bipedalism, and the Mosaic Evolution of the Helpless Infant” in Costly and Cute: Helpless Infants and Human Evolution. Edited by Wenda R. Trevathan and Karen R. Rosenberg. New Mexico: University of New Mexico Press. 67-86.
Dunsworth, Holly M., Anna G. Warrener, Terrence Deacon, Peter T. Ellison, and Herman Pontzer. 2012. “Metabolic hypothesis for human altriciality.”
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Dunsworth, Holly M. 2016. “The Obstetrical Dilemma Unraveled” in Costly and Cute: Helpless Infants and Human Evolution. Edited by Wenda R. Trevathan and Karen R. Rosenberg. New Mexico: University of New Mexico Press. 29-50.
Epstein, Herman T. 1973. “Possible metabolic constraints on human brain weight at birth.” The American Journal of Physical Anthropology 39:1, 135-136. https://doi.org/10.1002/ajpa.1330390114
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Taylor, Timothy. 2010. The Artificial Ape: How Technology Changed the Course of Human Evolution. New York: Palgrave Macmillian.
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