Evolution, Implications, and Selective Pressures of the Obstetric Dilemma

This entry was written by Sarah Partridge as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2.


The obstetric dilemma (OD) is a hypothesis that attempts to explain the complexity of Home sapiens (human) birth and why assistance is required to prevent or resolve complications that arise (Wells, DeSilva, Stock, 2012;). The OD affects female reproductive success and may have evolved from competing selective pressures affecting female pelvis morphology. Pressures on the pelvis that favour female reproduction will only affect the female pelvis, whereas pressures favouring thermal regulation and bipedal locomotion would affect both males and females. Various evolutionary perspectives have voiced how they believe the OD evolved and the factors contributing to it based on skeletal remains across the hominin lineage.

This entry discusses how selective factors have contributed to female pelvic morphology and offspring growth, and how this both causes and attempts to resolve the OD by finding a midpoint between the mother and the offspring’s fitness, how it evolved, and its future directions.

Main Text

Obstetric Dilemma

The OD was first proposed as competition between accommodating bipedal locomotion and birth of larger headed/brained neonates causing variation in female hominin pelvises (Ruff, 2017). But Wells et al (2012) loosely defined the OD as a difficult birth process, that may be directly related to the fitness of the mother. Upon further research and the recovery of more hominin fossils, it has been determined that other factors such as thermal regulation via heat loss and broad shoulders also contribute to evolution in female pelvic morphology. These factors sometimes have opposing effects on the pelvis, requiring selection of compromises between them (Gruss and Schmitt, 2015).

To reduce the OD, a few adaptions have evolved. The first being female pelvic diameter expanding marginally during birth, secondly the birth canal is a shape that allows for rotational birth, and lastly the neonate head is compressible (Wells, 2015). Smaller primates may also experience the OD, and they reduce it similarly to humans, excluding the rotational birth, suggesting it may be ancestral (Wells, DeSilva, Stock, 2012).

Skeletal Evidence

From the limited hominin pelvic fossil remains available it can be hard to determine the sex, making it difficult for evolutionary comparisons of male and female pelvic morphology, but it is attempted (Gruss and Schmitt, 2015).

An increase in brain size occurred across Homo, likely resulting in the need for maternal pelvic adaptions in order to birth the neonate. Wells (2015) suggested that Australopithecines had higher cephalon-pelvic disproportion (a condition involving the OD in which the foetal head is too large to fit through the pelvis) than Homo erectus, suggesting body size and crbsanial size changed in opposing ways around evolution of Homo. Rotational mechanisms of birth evolved to accommodate emergence of a large head and broad shoulders. Gruss et al (2015) and Wells et al (2012) investigated dimensions of the birth canal in non-human primates and humans. Non-human primates have a straight birth canal with a large diameter and posterior sacrum that does not require rotation for the head to emerge, but in some broad-shouldered species such as apes there can be rotation for the emergence of the shoulders. Humans have a cylindrical birth canal with varying angles and dimensions and a more intrusive sacrum that requires multiple rotations for the emergence of the head and shoulders. This suggests broadened shoulders are a factor that led to changes in the birth canal, allowing rotational birth to lessen the OD, and thereby increase the reproductive success of females.

Wells et al (2012) investigated evolutionary history of the variation of hominin OD by comparing pelvises across the hominin lineage. Pelvic fossils recovered of Australopithecus, Pleistocene human, and recent human populations suggest the maternal pelvis and foetal growth patterns show sensitivity to ecological factors such as diet and thermal environment rather than purely locomotion and birth factors. This implies there were long-term alterations in the magnitude of the OD across the hominin lineage (Wells, 2015). It has been found that a lower birthweight across populations occurs in hotter climates, where thermodynamics of the neonate/mother becomes inefficient and can lead to preterm birth. A wider and deeper pelvis increases efficiency of thermodynamics due to improved surface area/mass ratio, thereby females with narrower hips may be more prone to these complications, which should favour selection of wider pelvises in females.

Wells et al (2012) suggested a hypothesis in which early hominin pelvises had visible competition between locomotion and birth factors resulting in an increase of the OD, followed by the evolution of rotational birth mechanisms that eased this before new ecological stresses such as diet and thermal regulation intensified it again.

Possible Future of the Obstetric Dilemma

Before medical advancements, it was hard to limit or avoid complication caused by the OD. The invention of caesarean sections has allowed foreseeable complications such as cephalon-pelvic disproportion and breech neonates to be resolved (Wells, DeSilva, Stock, 2015). But if, hypothetically, the use of caesareans outweighed traditional birth, could this effect birthweight and the OD? For example, if the mother’s energy permits, the neonate head and brain become more develop, or the female pelvis becomes narrower as a result of less use.

Wells (2015) investigated whether the OD could be relieved if the neonate head and/or shoulders somehow became smaller. Theoretically this would reduce the OD, but the neonate would be born earlier and may thereby lack the amount of brain development required to survive once born. He then moved on to why female pelvises could not become larger. This explanation is simple, there are already many factors competing to shape morphology of the pelvis, and unless it outweighs the other factors there is not much to be done.


The OD is a compromise between the weighting of competing selective factors, that depend on how the environment and needs of hominins changed as they evolved. Therefore, the components of female individual’s pelvis give insight into the magnitude at which she may experience the OD, thereby affecting her reproductive success. Skeletal remains across the hominin lineage suggest bipedal locomotion, size of the neonate head/shoulders, diet and thermal regulation are factors that helped shape the pelvis and contributed to the OD. Remains also suggest the pelvis adapted to reduce the magnitude of the OD by widening during birth and using rotational mechanisms for emergence of the head and shoulders. Therefore, biological evolution has led to various adaption strategies to deal with the OD to improve female reproductive success.


Gruss L., Schmitt D., (2015), The evolution of the human pelvis: changing adaptions to bipedalism, obstetrics and thermoregulation. Philosophical Transactions B, 370(1663), 1-13

Ruff C., (2017), Mechanical Constraints on the Hominin Pelvis and the “Obstetrical Dilemma”. The Anatomical Record, 300(5), 946-955

Wells J., (2015), Between Scylla and Charybdis: renegotiating resolution of the ‘obstetric dilemma’ in response to ecological change. Philosophical Transactions B, 270(1663), 1-12

Wells J., DeSilva J., Stock J., (2012), The Obstetric Dilemma: An Ancient Game of Russian Roulette, or a Variable Dilemma Sensitive to Ecology?. Yearbook of Physical Anthropology, 55, 40-71

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