When did the modern human pattern of childbirth arise? New insights from an old Neandertal pelvis

Human birthing is difficult owing to a tradeoff between large neonatal brain size and maternal pelvic dimensions, which are constrained by aspects of bipedal biomechanics (1, 2). The net effect is that human neonatal head size closely matches maternal pelvic dimensions, unlike in our closest living relatives, the great apes, whose pelvic dimensions are larger than neonatal head sizes. This size relationship, along with a twisted birth canal shape, makes human parturition mechanically difficult and results in a unique pattern of “rotational” birth (Fig. 1). As in humans, and unlike great apes, monkey neonatal head size closely matches the mother's pelvic dimensions. Although birth rotation occurs in some monkeys, the rotation pattern is different from that in modern humans, with monkey neonates exiting the birth canal facing forward (1). Therefore, in both apes and monkeys, it is relatively easy for a mother to guide her infant out of the birth canal, keep the umbilical cord from wrapping around the neck, and extract mucous from the nose and mouth to facilitate breathing. All of these critical activities are much more difficult for a human mother, whose infant emerges facing backwards, and pulling on the infant from this position also risks serious neck injury. For these reasons, humans uniquely engage in assisted birth (obligate midwifery). Although unassisted birth, the norm for nonhuman primates, does occasionally occur in humans, assisted birth and its myriad social implications are the human norm cross-culturally (1).