There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form

Abstract

Compared to other primates, modern humans face high rates of maternal and neonatal morbidity and mortality during childbirth. Since the early 20th century, this "difficulty" of human parturition has prompted numerous evolutionary explanations, typically assuming antagonistic selective forces acting on maternal and fetal traits, which has been termed the "obstetrical dilemma." Recently, there has been a growing tendency among some anthropologists to question the difficulty of human childbirth and its evolutionary origin in an antagonistic selective regime.

The genetic architecture and evolutionary consequences of the human pelvic form

“…-log 10 (p) Chromosome pelvic inlet length: p = 5.3 × 10 -3 , odds ratio (OR) = 0.96; subpubic angle: p = 4.4 × 10 -4 , OR = 0.92) (Table S15). As a positive control, we examined another skeletal trait, leg-to-torso length, which we found to be significantly positively associated with walking speed (p = 2.97 × 10 -8 , OR = 1.08), in line with previous results and with mechanical modeling (6,7). These results provide empirical evidence that narrower birth canal proportions in humans are associated with increased walking speed (phenotypic association: between oblique pelvic inlet length and walking pace: p = 5.31 × 10 -3 , OR = 0.96, between subpubic angle and walking pace: p = 4.89 × 10 -4 , OR = 0.92).…”

“…This enables human infants to be born relatively earlier in development than their primate counterparts, consequently limiting the extent of brain growth before birth and ensures that the newborn can successfully traverse the birth canal during delivery. However, this hypothesis has been challenged and updated in recent years, as human gestational length and newborn size have been found to align with or exceed expectations for primates of our size, similar to the other great apes (14,(29)(30)(31) (see (6,7,32) for alternate usages and historical perspectives on the term "obstetrical dilemma").…”

“…The suite of adaptations for bipedality includes a reduction of the bi-acetabular distance, minimizing pelvic rotation during bipedal movement and consequently enhancing efficiency (5). This narrowing of the bi-acetabular distance results in a narrower birth canal, and is thought to stand in direct opposition to the birthing of children with significantly larger brains than our evolutionary predecessors (e.g., (6)(7)(8)(9)(10)(11)(12)(13)(14)). In the 1960s, this functional and evolutionary conflict was coined the "obstetrical dilemma" by Washburn (13).…”

“…In the 1960s, this functional and evolutionary conflict was coined the "obstetrical dilemma" by Washburn (13). In the six decades since then, the obstetrical dilemma has been a source of intense debate, and different studies have attempted to examine the validity of the hypothesis through empirical data (6,(14)(15)(16)(17). One area of contention centers on the relationship between pelvic shape and walking efficiency or walking speed.…”

Reconsidering the developmental origins of adult disease paradigm

“…Subsequent work has shown that our gestation is not short compared to other apes [ 54 , 55 ], while there is discussion over whether maternal energetic limits might also explain the typical duration of human pregnancy [ 55 , 56 ]. Nonetheless, maternal pelvic dimensions may still be subject to evolutionary constraint [ 57 ]. For example, the dimensions of the anteroposterior oval outlet and inlet are considered to have responded to contrasting selective pressures, associated with pelvic floor support and upright posture, respectively [ 58 , 59 ].…”