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Controversy Regarding the Obstetric Dilemma
by Dana Macfarlane

The evolution of human pelvic morphology has 
been extensively researched for several decades, with 
most studies referring back to an article published 
almost sixty years ago. In 1960, S. L. Washburn 
proposed the obstetric dilemma (OD) hypothesis in 
order to describe the evolution of the human pelvis. 
The hypothesis proposes that stabilizing selection of 
the human female pelvic morphology results from 
the conflicting demands of bipedal locomotion 
and the birth of large-headed neonates (Fischer & 
Mitteroecker, 2015). Thus, while a narrow pelvis would 
optimize bipedal locomotion, a wide pelvis would 
accommodate large-headed neonates. Washburn 
(1960) thus suggested the dilemma between the two 
opposing factors would be ameliorated by stabilizing 
selection, allowing for both bipedalism and large-
headed neonates. However, recent research suggests 
that the obstetric dilemma hypothesis is incorrect 
or is insufficient for the complex description of the 
selective pressures acting on the pelvis throughout 
the evolution of hominids. Some even propose that 
there is actually very little sexual dimorphism of the 
human pelvis (Beti et al., 2013; Gruss & Schmitt, 
2015). The question, therefore, becomes: is there 
sexual dimorphism in the human pelvis and, if so, 
why? The disagreements are important to review 
because they indicate that factors like variability and 
environmental effects may be able to provide greater 
insight into the evolution of humankind. 

The obstetric dilemma was developed in order 
to describe the evolution of the human pelvis as 
it pertains to childbearing and not only makes a 
hypothesis, but also predicts aspects impacted by the 
dilemma such as pelvis size. The aforementioned two 
opposing forces—a narrow pelvis to optimize bipedal 
locomotion and a wide pelvis to accommodate large-
headed neonates—thus create a stable morphology 
for the female pelvis, which would lead one to 
predict less variability amongst female pelvises 
than that of males (Gruss & Schmitt, 2015). While 
the obstetric pressure is derived from large-headed 

babies, selecting for wide canals, the bipedal 
locomotion pressure would select for a narrower 
pelvis in this instance, as the OD claims a wider and 
more feminine pelvis has higher locomotor costs 
(Fischer & Mitteroecker, 2015; Washburn, 1960). 
Additionally, the OD proposes that the size of the 
fetal brain is restricted by the locomotor pressures 
acting on the pelvis. This means that the stabilizing 
selection predicted by the OD prevents the pelvic 
canal from becoming larger because this would 
compromise locomotor needs (Dunsworth et al., 
2012). The dilemma is also not unique to humans, 
as other small-bodied primates like macaques have 
a tight fit in the birth canal and have rotation during 
birth (Trevathan, 2015). The obstetrics for this 
hypothesis deal only with parturition and the set up 
for birth and do not comment on other aspects such 
as metabolism or proportions. 

Testing of the OD hypothesis has been done on 
the biomechanics of pelvic morphology in humans 
in order to assess whether locomotor costs are as 
great as the hypothesis implies. The consequence 
of dissimilar pelvic morphologies on bipedal 
locomotor costs can be explored by focusing on the 
hip abductor muscles (gluteus minimus and gluteus 
medius) and the force activation of these muscles 
(Warrener et al., 2015). These muscles can be tested 
by having participants walk and run at set speeds 
on treadmills while measuring components such 
as oxygen consumption to estimate hip abductor 
cost. A limitation of this method is that individuals 
do not all have the same base walk and run speeds 
(Warrener et al., 2015). While the notion of sexual 
dimorphism of the human pelvis is widely accepted, 
differences between the sexes’ energetics in regards 
to the mechanics of hip abductor muscles appear to 
be negligible (Dunsworth et al., 2012; Warrener et al., 
2015). This result is contrary to the OD hypothesis, 
which states that females will have higher locomotor 
costs due to the wider pelvis (Washburn, 1960). 
Although slight variations in locomotor dynamics 



Controversy Regarding the Obstetric Dilemma

Aisthesis      Volume 9,  201828

have been seen, they are not significant enough to 
suggest higher locomotor costs in females and could 
possibly be indicative of counteractive measures 
to energetic costs, wherein the muscle is situated 
differently than the males but works more effectively 
with a wide pelvis (Dunsworth et al., 2012; Warrener 
et al., 2015). 

The evolutionary cause of sexual dimorphism 
in human pelvic morphology is uncertain. When 
pelvic landmarks are measured across a variety of 
populations, there are multiple explanations within 
the anterior and posterior spaces (Brown, 2015). 
Anterior spaces include landmarks such as the 
inferior ischial tuberosity and the inferior pubis, 
while posterior spaces include the pelvic inlet and 
the sacrum. The spaces can be analyzed using body 
mass as a proxy for biomechanical selective pressures 
and sex as a proxy for obstetric selective pressures 
(Brown, 2015). Body mass has the potential to be 
used as a proxy for biomechanics in this instance 
because of the impact of weight on locomotion and 
the proper function of muscles. Comparing results, 
sections of both spaces show signs of variation 
due to biomechanics, while other sections within 
each space show signs of variation due to obstetric 
pressures (Brown, 2015). These results are difficult 
to interpret in terms of the obstetric dilemma 
because they do not illustrate definitive spaces for 
obstetric and biomechanical pressures within the 
pelvis; however, these results loosely support the OD 
because of the influence from both locomotion and 
obstetrics (Brown, 2015). Tests on the physiology 
of the human pelvis indicate that there is variation, 
but that this is not entirely due, or due at all, to the 
biomechanic energetics of humans. Pinpointing the 
magnitude of locomotor costs on the anatomy of the 
pelvic area seems to be more complex than originally 
thought, especially when incorporating data from 
both muscles and bone.

According to the OD, the female pelvis would 
be less variable in morphology compared to males 
due to the stabilizing effect of the opposing factors, 
bipedal locomotion and large-headed neonates. 
Another area of interest when investigating the 
obstetric dilemma is the variation of the pelvis 
within and between populations because the 
stabilizing selection clause of the hypothesis would 
be upheld by different levels of variation between 

the sexes (Kurki, 2013). In order to test this, pelvic 
landmarks must be measured and compared to other 
individuals in the sample, with the analysis focused 
on the amount of variation within a sex and between 
the sexes. However, this assumption has been refuted 
by evidence which suggests there is no significant 
difference in skeletal variability levels between the 
sexes, even across diverse populations (Betti et al., 
2013; Kurki, 2013; Kurki & Decrausaz, 2016). Not 
only this, but the pelvic canal was also found to be 
just as variable as other parts of the body, such as 
the limbs, for the same individuals (Kurki, 2013; 
Kurki & Decrausaz, 2016). It should also be noted 
that variability of the pelvis can be found amongst 
monozygotic twins, and not all areas varied the same 
(Sharma, 2002). Because of findings like these, some 
propose that any evident variability between the 
sexes is not produced by obstetric constraints (Gruss 
& Schmitt, 2015).

Variability of the pelvis has also been linked to 
body proportions in individuals. While the shape and 
size of the pelvis have not been found to be uniform 
within a population, they have shown correlations 
to the stature and head size of individuals (Fischer 
& Mitteroecker, 2015). When pelvic landmarks 
are compared between persons, tall height is 
connected to narrow pelvises, and large heads are 
connected to wider canals, regardless of sex (Fischer 
& Mitteroecker, 2015). Additionally, humans have 
broad shoulders which further complicate birth and 
require a large exit much like the head (Trevathan & 
Rosenberg, 2000). However, the correlation between 
birth canal proportions and body proportions does 
not imply causation (Gruss & Schmitt, 2015).   

A factor known to cause differences within 
populations over time is neutral variation, which 
would imply that the mutation rate of separate 
groups would be unaffected by outside forces or that 
no selection was occurring (Betti et al., 2013). The 
variance of the os coxae of the pelvis is found to be 
comparable to the pattern of neutral genetic markers, 
which suggests that neutral evolution, not stabilizing 
selection, would be a prominent factor in shaping 
this area of pelvic morphology of local populations 
today (Betti et al., 2013). This would imply that the 
variation seen among humans is random change 
and, if certain pressures shaped it in the past, those 
pressures are no longer notable. If neutral variation 



Controversy Regarding the Obstetric Dilemma

Aisthesis      Volume 9,  201829

was indeed the main cause for variation in the pelvis 
today, this would suggest other factors besides pelvic 
morphology are important for the determination of 
human childbearing (Betti et al., 2013).    

Environmental components acting on the 
plasticity of the pelvis could be the cause of the 
observed variations which contest the obstetric 
dilemma (Dunsworth et al., 2012; Kurki, 2013; 
Kurki & Decrausaz, 2016; Sharma, 2002). Variation 
in the pelvic morphology of monozygotic twins 
was found in a great deal of the pelvic area and 
demonstrates the importance of external factors for 
the development of individual morphology (Sharma, 
2002). The range of lifestyles of study participants is 
a difficult additional variable to account for and may 
include aspects such as physical activity, diet, and 
experiences, all of which can alter the morphology of 
the pelvis (Kurki & Decrausaz, 2016). Furthermore, 
these aspects may impact natural hormone levels, 
which may themselves be an important variable in 
regard to the obstetric dilemma (Betti et al., 2013). 

There is also the proposition that the main 
determining factor for the evolution of human 
childbearing is the limitation of the maternal 
metabolism (Betti et al., 2013; Dunsworth et al., 2012; 
Gruss & Schmitt, 2015; Trevathan, 2015). Pregnancy 
alters the maternal process of thermoregulation, 
or the act of obtaining homeostasis; as the fetus 
develops, it requires more energy from the mother 
and, by nine months, the maternal metabolism 
cannot keep up with the demands of the fetus 
(Dunsworth et al., 2012; Trevathan, 2015). After 
birth, the child can be fed on breast milk, which is 
less energetically taxing than development in the 
womb (Dunsworth et al., 2012). Additionally, it has 
been proposed that maternal metabolic constraints 
are more important than locomotion for analyzing 
current pelvic morphology variation; while 
locomotion may have been a pressure for selection 
in the past, this evidence would suggest a shift in 
selective pressures with emphasis on the limitations 
of the maternal metabolism (Dunsworth et al., 2012; 
Gruss & Schmitt, 2015). Dubbed the Energetics 
and Gestation of Growth (EGG) hypothesis, this 
new rationale proposes that the fetal brain size is 
constrained by the maternal metabolism and that the 
female pelvis adapted to the fetal brain (Dunsworth 
et al., 2012). The concept of the EGG stems from 

the lack of evidence for locomotor costs and brain 
expansion constraints from pelvic mechanics. This 
hypothesis thus suggests that the fetal brain size is 
not limited by the pelvis, but that the female pelvis 
and the fetal brain are both limited by the maternal 
metabolism; this would also suggest that the maternal 
metabolism limits the length of gestation, which in 
turn limits fetal brain size (Dunsworth et al., 2012). 
The obstetric dilemma details that the mechanical 
issues brought on by birthing large-headed neonates 
while maintaining optimal bipedal locomotion shape 
the morphology of the human pelvis (Washburn, 
1960). From the thermoregulation standpoint, pelvic 
morphology is heavily influenced by the energetics 
of the mother and not on the mechanics of childbirth 
(Dunsworth et al., 2012; Gruss & Schmitt, 2015). 

Research into the sexual dimorphism of the 
human pelvis has shown that there are numerous 
potential pressures that influence morphology. 
Washburn laid out the obstetric dilemma as an 
explanation for human pelvic morphology variation, 
although the predictions made in this hypothesis 
have received criticism and have been met with 
conflicting evidence. His ideas about the importance 
of the opposing forces of obstetrics and locomotion 
have been argued against, citing other factors such as 
neutral variation and thermoregulation (Betti et al., 
2013). In summary, the obstetric dilemma has been 
a foundation for decades of research in the field of 
evolutionary biology, and the findings presented in 
this review only help to expand the understanding of 
the evolution of human pelvic morphology.
      
      
 



Controversy Regarding the Obstetric Dilemma

Aisthesis      Volume 9,  201830

References
Betti, L., von Cramon-Taubadel, N., Manica, A., &

Lycett, S. J. (2013) Global geometric 
morphometric analyses of the human pelvis 
reveal substantial neutral population history 
effects, even across sexes. PLOS ONE, 8(2): 
e55909. https://doi.org/10.1371/journal.
pone.0055909

Brown, K. M. (2015). Selective pressures in the
human bony pelvis: Decoupling sexual 
dimorphism in the anterior and posterior spaces. 
American Journal of Physical Anthropology, 
157(3), 428-440. doi:10.1002/ajpa.22734

Dunsworth, H. M., Warrener, A. G., Deacon, T.,
Ellison, P. T., & Pontzer, H. (2012). Metabolic 
hypothesis for human altriciality. Proceedings of 
the National Academy of Sciences of the United 
States of America, 109(38), 15212–15216. http://
doi.org/10.1073/pnas.1205282109

Fischer, B., & Mitteroecker, P. (2015). Covariation 
between human pelvis shape, stature, and head 
size alleviates the obstetric dilemma. Proceedings 
of the National Academy of Sciences, 112(18), 
5655-5660. doi: 10.1073/pnas.1420325112

Gruss, L. T., & Schmitt, D. (2015). The evolution of
the human pelvis: Changing adaptations to 
bipedalism, obstetrics and thermoregulation. 
Philosophical Transactions of the Royal Society 
B: Biological Sciences, 370(1663). doi:10.1098/
rstb.2014.0063

Kurki, H. K. (2013). Skeletal variability in the pelvis 
and limb skeleton of humans: Does stabilizing 
selection limit female pelvic variation? American 
Journal of Human Biology, 25(6), 795-802. 
doi:10.1002/ajhb.22455

Kurki, H. K., & Decrausaz, S. (2016). Shape variation
in the human pelvis and limb skeleton: 
Implications for obstetric adaptation. American 
Journal of Physical Anthropology, 159, 630-638. 
doi:10.1002/ajpa.22922

Sharma, K. (2002). Genetic basis of human female
pelvic morphology: A twin study. American 
Journal of Physical Anthropology, 117(4), 327-
333. doi:10.1002/ajpa.10055

Trevathan, W., & Rosenberg, K. (2000). The shoulders
follow the head: Postcranial constraints on 
human childbirth. Journal of Human Evolution, 
39(6), 583-586. doi:10.1006/jhev.2000.0434

Trevathan, W. (2015). Primate pelvic anatomy and
implications for birth. Philosophical Transactions 
of the Royal Society B: Biological Sciences, 
370(1663). doi:10.1098/rstb.2014.0065

Warrener, A. G., Lewton, K. L., Pontzer, H., &
Lieberman, D. E. (2015). A wider pelvis does 
not increase locomotor cost in humans, with 
implications for the evolution of childbirth. PLoS 
ONE, 10(3), e0118903. http://doi.org/10.1371/
journal.pone.0118903

Washburn, S. (1960). Tools and human evolution. 
Scientific American, 203(3), 62-75. Retrieved 
from http://www.jstor.org/stable/24940615

Wittman, A. B., & Wall, L. L. (2007). The
evolutionary origins of obstructed labor: 
bipedalism, encephalization, and the human 
obstetric dilemma. Obstetrical & Gynecological 
Survey, 62(11), 739-748. doi:10.1097/01.
ogx.0000286584.04310.5c


