childbearing and childbirth in females. Adaptations to ensure live birth Early human ancestors, hominids, originally gave birth in a similar way that non-human primates do because early obligate quadrupedal individuals would have retained similar skeletal structure to great apes. Most non-human primates today have neonatal heads that are close in size to the mother's birth canal, as evidenced by observing female primates who do not need assistance in birthing, often seeking seclusion away from others of their species. In modern humans, parturition (childbirth) differs greatly from the rest of the primates because of both pelvic shape of the mother and neonatal shape of the infant. Further adaptations evolved to cope with bipedalism and larger craniums were also important such as neonatal rotation of the infant, shorter gestation length, assistance with birth, and a malleable neonatal head. Neonatal rotation Neonatal rotation was a solution for humans evolving larger brain sizes. Comparative zoological analysis has shown that the size of the human brain is anomalous, as humans have brains that are significantly larger than other animals of similar proportions. Even among the great apes, humans are distinctive in this regard, having brains three to four times larger than those of chimpanzees, humans' nearest relatives. Although the close correspondence between the neonatal cranium and the maternal pelvis in monkeys is also characteristic of humans, the orientation of the pelvic diameters differs. On average, a human fetus is nearly twice as large in relation to its mother's weight as would be expected for another similarly sized primate. The extremely close correspondence between the fetal head and the maternal pelvic dimensions requires that these dimensions line up at all points (inlet, midplane, and outlet) during the birth process. During delivery, neonatal rotation occurs when the body gets rotated to align head and shoulders transversely when entering the small pelvis, otherwise known as internal rotation. The fetus then rotates longitudinally to exit the birth canal, which is known as external rotation. In humans, the long axes of the inlet and the outlet of the obstetric canal lie perpendicular to each other. This is an important mechanism because growth in the size of the cranium as well as the width of the shoulders makes it more difficult for the infant to fit through the pelvis. This enables the largest dimensions of the fetal head to align with the largest dimensions of each plane of the maternal pelvis