Squished Part 2: Boney Bits

In this four-part series, I am drawing on decades of research to discredit the pernicious myth of the Obstetrical Dilemma. In the first part, I explain what the Obstetrical Dilemma is, where (and when) it originated and what some of the consequences of the myth are. In this second part, I take on the skeletal side of the theory, namely women’s pelvises and the skull of the infant, the combination of which is also referred to as fetopelvic morphology. In part three, I move on to infant development at birth to dispell this idea that human infants are born prematurely. And finally, in part four, I explain how the infant carrier has influenced the shape (literally) of the modern human by providing support for poorly clinging big headed hominids long before modern humans were on the scene.

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 really 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, conjoined twins, etc. For these and many other evidence-based, medically-indicated reasons unrelated to fetopelvic morphology, c-sections can be life-saving. However, being a bipedal female and pregnant is not in itself a medical indication for surgical delivery and c-sections are serious surgical procedures with serious risks. /steps off soapbox/


The foundation of the Obstetrical Dilemma (OD) –aside from a delightful combination of sexism and racism– is that in order to walk efficiently bipedalism requires narrow hips, too narrow to give birth to a fully developed fetus, if at all. As noted in part one, it’s based in the idealization of the male form, that is, that men’s narrower pelvis make him a more efficient walker and runner than a female. This theory was repeated in the scientific literature as a basis for the OD for almost 70 years Without. Ever. Being. Tested.

“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. 2015

That is until Warrener and her colleagues set to the task, fully expecting to support the OD hypothesis and even in the face of their own data to the contrary– they try desperately to cling to the myth that neonates are regularly too big to fit through the birth canal, though in reality, it’s an extremely rare occurrence owing to disease or deformation (Dunsworth, 2016).

“[…] 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

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 (see the first quote from Warrener, above) acceptance of an idea for a long time makes a fact. A lot of sexist ideas had wide-spread, long-term acceptance.

“[…]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 (emphasis added)

e28dffa3c5396b5064779e3f589cf255I mean, the entire OD is a correlational fallacy but it was surprising to find professional scholars making the assumption that OD was true because other people have said it was true without checking the data (or noting the lack thereof). And you know what they say about assuming, right?

Bipedalism & Big Babies

As noted in part one, the Obstetrical Dilemma hypothesis came about when hospital birth had been an established convention for around a generation, at a time when anthropologists believed that black people were a separate species of primate from white people (Schultz, 1949) and that modern humans sprang from man-apes (Washburn, 1960). Modern (white) humans were considered a sudden exceptional species with unique problems giving birth which required obstetrical technology and technique to manage. But we have learned a lot since then (at least, some of us have) for example, that bipedalism goes back far beyond our genus, over four million years, to the species Australopithecus afarensis.

One of the more famous specimens of A. afarensis was named Lucy, so we’ll refer to her species by that name. Lucy was fully bipedal, though with proportionally wider hips than modern humans (see the video). Her species seems to be the first to have big babies, that is, neonates 5-6% of the mother’s mass (infant:mother mass ratio, or IMMR) which is the same proportion modern human infants have; while other great apes have IMMRs of 2-3%, similar to Lucy’s evolutionary predecessors Ardipithecus (DeSilva, 2011). This marks a split in our evolutionary life history: for some reason our bipedal ancestors started having much larger babies than our non-bipedal ancestors or cousins.

“Based on the size of the birth canal in australopith pelves […] and on regression-based estimates of brain size in australopith infants, birth was a challenging event early in human evolution. Although it is not clear whether australopiths experienced the corkscrew rotation through the birth canal that characterizes most human births, the cephalopelvic index alone would suggest that australopiths labored during birth. Furthermore, given the width of ape and human shoulders, even when the head emerged from the birth canal, the wide shoulders still posed a problem.” DeSilva, 2016.

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.

So the history for proportionately large neonates being gestated and passing through narrow bipedal pelvises go back in evolution a long way, long before modern humans evolved, therefore it’s not something unique to the human species. And, there are other primates, who are not bipedal, have what would seem an impossibly tight fit between the fetal head and maternal pelvis, yet they still manage to give birth.

“In all callitrichid species studied here (Callimico goeldii, Callithrix jacchus, Cebuella pygmaea), neonate heads are significantly larger than birth canal dimensions.” Ponce de Leon and Zollikofer, 2016

Selection Pressures & Fetal Skulls

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.

“The female form is the gateway to further evolution.” Dunsworth, 2016.

Thanks to Warrener’s research, we know that narrower hips provide no benefit for walking or running efficiency. So if narrower hips are truly a constraint for fetal size resulting in dangerous birth to an underdeveloped baby, then why didn’t hominin pelvises remain at a wider state? The simple answer is that the pelvis doesn’t constraint fetal size.

Proponents of OD 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 seems the author’s views on childbirth affect the fit. Perhaps in line with the perception of the author, the diagrams fail to show the movement of birth; of the pelvic joints, 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 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 women 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 that allow for the rapid brain growth in the years after birth, they finally fuse in during when the person has reached adulthood. Having plates of bone rather than a solid skull was not an adaptation intended to ease birth, besides, where the brain is squishy the shoulders are fairly rigid and they need to pass too.

It seems as though the human pelvis is overrated when it comes to its influence on bipedal efficiency and fetal size. What if the pelvis isn’t a constraint to birth or gestation at all? What if the pelvis is just there to support the trunk and internal organs, and gets out of the way for birth? And what if sexual dimorphism of hip width is just a means of ensuring COM remains steady through pregnancy?


During pregnancy, all of the connective tissues relax, including the joints of the pelvis– which can be a huge pain, sometimes requiring a brace to manage– this spreading is a means of helping the body support the growing fetus. Shoe sizes might even go up. During birth, the spread is even more dramatic as cervix softens and dilates (with absolutely no respect for the clock, so if you don’t want hands up your va-jay-jay you say NO. It’s not a good indication of labor progression.) The vagina too opens along with the other pelvic floor muscles– provided that the laboring woman feels safe and, bonus, loved (any mammal will have a smoother labor when they feel safe).  It’s the same as during sex when the vagina tents and believe it or not, women have reported orgasmic birth experiences. Now, I’m not here to tell anyone that labor and birth aren’t painful– I actually think that laboring for a social animal like us is a way of gathering support, ensuring postnatal provisioning (i.e. bring that mama food and beverages). But I will say that women who are in the kind of environment where they can reach orgasm during birth probably feel more safety and love than a frightened woman surrounded by aggressive strangers pointing at the clock being told she’s failing to progress.

As noted above, the pelvis is jointed and that those joints loosen during pregnancy and more so during labor– if they are allowed to mentally and physically. A frightened woman’s pelvic muscles will remain tight, restricting the spread of the pelvic bones and the descent of the fetus. This is a survival mechanism, helping her to escape to a safe place before giving birth, like any mammal. But say, the laboring woman feels safe, ready to go, but then is instructed to lie on her back, or put weight on her pelvis– that pelvic loosening for outward expansion gets pushed inward, causing pain and narrowing the outlet.

gaskin-maneuverA woman in a position that impedes the movement of her pelvis or feels threatened is more likely to have her baby’s shoulder stuck in the birth canal. This is one of the reasons for the Gaskin Maneuver to avoid or treat shoulder dystocia is so effective. Getting the laboring woman to her hands and knees allows the free movement of the pelvis while moving the muscles of the pelvic floor. The problems associated with shoulder dystocia occur when women are put in a position that doesn’t allow for pelvic movement and/or when an impatient or underskilled birth attendant pulls on the baby’s head which can cause a brachial plexus injury. Skilled birth attendants understand that birthing in an upright position is ideal because works with gravity, allows the pelvis to open, and helps relax the pelvic floor while tightening the core muscles. Unfortunately, there are self-serving doctors who put their comfort before the well-being of their patients.

“Another barrier for many nurses (33.6 percent) was that patients and physicians were ‘not open to the use of upright positions.’ As one nurse stated, ‘many of the doctors prefer to have patients pushing in low Fowler’s’ and ‘the obstetricians and family doctors want to deliver in the easiest position for them.'” Gilder, et al. 2002

Forcing a laboring woman into the lithotomy position or even “Low Fowler’s” position (on her back, knees to chest) to give birth for the convenience of the attendant should be a criminal offense. Even a woman with an epidural or paralysis can be helped into and supported in a position that doesn’t impede her pelvis’s ability to move (Gilder, 2002), so there is no excuse for not allowing a woman to give birth regardless of medical provider’s (or researchers) expectations of her ability to hold a posture.

“Although the squatting position may be optimal for delivery in that it has been shown to increase intra‐abdominal pressure and increase the diameter of the pelvis very few women, at least in most western societies, have the stamina to remain in this position for the length of time usually required to deliver a child.” Trevathan and Rosenberg, 2003.

You are very much allowed (and encouraged!) to throw objects at the last quote, because my goodness, how presumptuous and rude. Childbirth is basically one long lesson in the extraordinary stamina of women when they are treated with respect, love, and, you know, allowed to eat and drink.


Unlike what so many people, including researchers, believe– having birth attendants is not universal. It’s not a biological necessity, it’s cultural. There are cultures in which the laboring woman goes off by herself to give birth and returns to the community with her newborn (Belaunde, 2000). While authors like Taylor, bias by his own culture, imagines the earliest modern humans giving birth in a shiny obstetrical ward complete with bleeping machines in the grasslands of Africa +200,000 years ago (see part one for the quote), in some western nations, there is a movement of “unassisted birth” in which women choose to give birth without medically licensed attendants, and sometimes without any attendants at all. Though, I would argue that the presence of online communities is a form of birth attendants, supporting the woman through pregnancy and preparing her for labor and birth. Then there are the women around the world intending to have medical attendants either in the hospital, birth center, or at home who give birth so fast that they do it alone. And of course, the millions of women living in poverty who find themselves laboring without access to birth attendants or even basic sanitation.

Are these women the exception or the norm amongst their species? What is more likely: that the survival of big-headed hominins for the past four-million-years was down to the luck of an exceptional few women with unusually wide pelvises and small babies? And, again, if that is so, why weren’t those traits selected for?

I think that it is more likely that the medical model of birth and the obstetrical dilemma are culturally derived. The difficulties in birth, so-called “obstructed labor” or “cephalopelvic disproportion” are the result of the culture that created the OD hypothesis, namely, women are told to give birth on their backs (or that they can’t give birth vaginally at all) by professionals operating under the influence of the Obstetrical Dilemma. The following is the introduction to a paper on obstructed labor in 2007 by a doctoral student in anthropology and a professor of obstetrics At Washington University in St. Louis, Missouri:

“Childbirth in humans is difficult because the dimensions of the mother’s pelvis are relatively small and the fetus—particularly the fetal head—is large. Humans are thus predisposed to develop cephalopelvic disproportion, obstructed labor, and the catastrophic obstetric complications that can result when this process is unrelieved: uterine rupture, fistula formation, and the myriad injuries of the “obstructed labor injury complex”. Compared with the great apes to whom we are most closely related, human birth is uniquely slow and precarious (3–5). Thus, Sherwood Washburn referred to the human “obstetric dilemma” resulting from the shrunken dimensions of the human birth canal mandated by the mechanical requirements of upright bipedal locomotion and the evolution of progressively larger human brains.” Wittman and Wall, 2007.

Medical professionals working under this misapprehension think women are “predisposed” to obstructed labor based on their physiology, without any consideration that the way they expect their patients to labor and give birth could be the cause of obstructed labor. Instead of working with laboring women, obstetricians, especially less experienced obstetricians will jump to a cesarean even before labor begins, often based on estimates of the fetal size via ultrasound, a method which has “significant error levels” of overestimating fetal weight contributing to unnecessary c-sections (Milner, 2018).

“More experienced doctors perform fewer of these procedures than do their younger colleagues. Labor and birth position in a restrictive hospital setting (e.g. not taking advantage of the effect of gravity and relaxin, and often working against them), as well as assistance from an obstetrician rather than a midwife, seem to contribute to the difficulty women face in childbirth and recover.” Dunsworth, 2017.

If doctors are taught that the female body is inherently flawed then obstetricians will “heal with steel”. The obstetrical dilemma is used to justify cutting women, either in the form of the episiotomy or in surgical delivery. This is why I call OD a pernicious myth– episiotomies lead to 3-4 degree tearing, cut through skin and muscle, may lead to incontinence and chronic pain, and the only benefit is to the doctor who prefers to sew a straight cut rather than repair multiple random shallow tears (Gün, 2016). Healthy women are three times more likely to die from cesarean than vaginal birth (Mascarello 2017), and they risk the loss of fertility, chronic pain, and increased risk of uterine rupture during subsequent pregnancies (especially if their labor is induced or augmented with Pitocin).

This isn’t just an academic debate between scholars, this is a hypothesis that has led to higher maternal mortality rates because it presumes that cutting fetuses out of the womb is safer than risking it getting stuck in a too-small pelvis. And this model of maternity care is being exported to developing nations where more women will face the consequence of this sexist and wholly inaccurate hypothesis.

To recap: pelvic width does not help or hinder bipedal walking or running; bipedalism and large neonates pre-existed modern humans for millions of years, and narrower pelvises were selected for even as fetal head size increased; the pelvis changes shape to help support the center of mass of a pregnant woman and can expand during birth due to relaxin that also softens the cervix; fetal skulls mold because they can not because they have to; the pelvis does not constrict the brain growth of the fetus as within normal range of pelvic size differences modern human women could give birth to fetuses with x4 as large heads.

Take home: Human bodies are perfectly adapted to gestating and giving birth to human babies. Difficulties in birth are due to individual and cultural causes, not a species-wide maladaptation for bipedal efficiency. In part three, I will cover why human babies are born when and the way they are (just as they should be) without the need for the Obstetrical Dilemma hypothesis for an explanation of their dependency. 

Until next time… try to have a good laugh, this is some heavy sh*t.



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.”
PNAS 109:38, 15212-15216; DOI:10.1073/pnas.1205282109

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

Gilder, Kathy, Linda J. Mayberry, Susan Gennaro, and Donna Clemmens. 2002. “Maternal
Positioning in Labor With Epidural Analgesia.” Journal of Obstetric, Gynecologic, and Neonatal Nursing: AWHONN Lifelines 6:1, 40-45.

Gün, İ., Doğan, B., & Özdamar, Ö. 2016. “Long- and short-term complications of episiotomy.” Turkish Journal of Obstetrics and Gynecology, 13:3, 144–148. http://doi.org/10.4274/tjod.00087

Mascarello KC, Horta BL, Silveira MF. 2017. “Maternal complications and cesarean section without indication: systematic review and meta-analysis.” Revista de Saúde Pública. 51:105. doi:10.11606/S1518-8787.2017051000389.

Milner, J and J Arezina. 2018. “The accuracy of ultrasound estimation of fetal weight in comparison to birth weight: A systematic review.” Ultrasound 26:1, 32-41. doi: 10.1177/1742271X17732807.

Michaelson, Karen L., et al. 1988. Childbirth in America: Anthropological Perspectives. Massachusetts: Bergin & Garvey Publishers, Inc.

Mitford, Jessica. 1992. The American Way of Birth. New York: Dutton (Penguin Group).

Ponce de Leon, Marcia, and Christoph P.E. Zollikofer. 2016. “Primate Birth at the Extremes: Exploring Obstetric and Metabolic Constraints” 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. 51-67.

Schultz, Adolph H. 1949. “Sex Differences in the Pelves of Primates.” American Journal of Physical Anthropology 7:3, 401-424.

Taylor, Timothy. 2010. The Artificial Ape: How Technology Changed the Course of Human Evolution. New York: Palgrave Macmillian.

Wittman, Anna Blackburn, and L. Lewis Wall. 2007. “The Evolutionary Origins of Obstructed Labor: Bipedalism, Encephalization, and the Human Obstetric Dilemma.” Obstetrical and Gynecological Survey 62:11.

Rosenberg, Karen and Wenda Trevathan. 2003. “Birth, obstetrics and human evolution.” An International Journal of Obstetrics and Gynaecology 109:11, 1199-1206

Warrener, Anna G., Kristi L. Lewton, Herman Pontzer, and Daniel E. Lieberman. 2015. “A Wider Pelvis Does Not Increase Locomotor Cost in Humans, with Implications for the Evolution of Childbirth.” PLoS ONE 10:3, e0118903.

Whitcome, Katherine K., Liza J. Shapiro, and Daniel E. Liberman. 2007. “Fetal Load and the Evolution of Lumbar Lordosis in Bipedal Hominins.” Nature 450, 1075-1080.

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