Humans are not unique in having difficult births: A comparative primate analysis

Human childbirth is regarded as uniquely difficult among primates, due to a tight cephalopelvic fit thought to result from an evolutionary trade-off between adaptations to bipedal locomotion and increasing brain size. This interpretation, however, may be an artefact of anthropocentric measurements that underestimate birth challenges in non-human primates. Here, we re-evaluate cephalopelvic proportions using species-specific three-dimensional data of the pelvic inlet, along with neonatal cranial dimensions in both sinciput and face presentation from a broad sample of extant primates, demonstrate that a tight fit is not exclusive to humans, and that higher cephalopelvic proportions occur in various other primates. The results reveal that maternal body size is a key variable in understanding primate variation in fetopelvic fit: smaller species have relative larger neonates as well as a relatively smaller pelvic canal, leading to high cephalopelvic proportions. Extremely tight cephalopelvic fit occurs in species with proportionately larger neonates, smaller pelves or a combination of both. The latter is the case in humans, producing the tightest fit among extant apes, but a similar combination of factors explains much more extreme cephalopelvic proportions in other species, revealing a diversity of obstetrical dilemmas across primates.