The mechanism of labor in nonhuman primates: A look inside

While in humans, the flexed position of the fetus and its rotating course down the birth canal are well documented, in other primates the mechanism of labor is unknown. Despite the lack of comparative data, it is commonly assumed that the human obstetric mechanism is unique, and anthropologists have disputed when and why the transition to the modern human mechanism occurred. The purpose of this study is to establish the mechanism of labor in two species of nonhuman primates. The data in this study consist of a series of radiographs taken throughout labor in unanesthetized, unrestrained, spontaneously laboring Papio anubis (n = 4) and Saimiri boliviensis (n = 7), pelvimetric radiographs, and neonatal cephalometry. In both species, the fetus normally presents in extension, with the face or brow dilating the cervix, and the head being delivered with the mentum anterior. Contra popular assumption, fetal rotation during labor was observed, and was analogous to rotation in human face presentations. Dramatic pelvic deformation, but not fetal head molding, was recorded. I suggest that previously published cephalopelvic ratios of nonhuman primates do not accurately represent the obstetric situation. Here, measurements of the actually constrictive diameters of the nonhuman primate pelvis and functionally significant measures of the fetal head are presented. The implications of this study for the obstetric evaluation of fossil hominid pelves are discussed.