Human birth is a social process.

Rosenberg, K., & Trevarthan, W. (2002). Birth, obstetrics, and human evolution. BJOG: an International Journal of Obstetrics and Gynaecology, 109, 1199-1206.

"This emergence pattern accounts for many unique aspects of human maternal behavior at birth. For humans birth involves other people. Unlike non-human primates, which seek solitude at this time, human mothers actively seek assistance in childbirth.... Like non-human mothers, human mothers often squat during delivery, although they assume a wide range of other postures... in general the upright posltion (sitting, squatting or standng) has been shown to result in a shorter second stage of labour, better infant outcomes, and fewer negative sequalae for the woman..."

"Because the human fetus emerges from the birth canal facing in the opposite direction from its mother, it is difficult for the mother, whatever her position, to reach down, as non-human mothers often do, to clear a breathing passage for the infant or to remove the umbilical cord from around its neck. If the human mother tries to assist in delivery by guiding the infant from the birth canal, she risks pulling it aganst the body's angle of flexion, possibly damaging the infant's spinal cord, brachial nerves and muscles." "..although giving birth alone is mentioned as a cultural ideal among the !Kung.. most women have their mothers, sisters or other women with them....Human birth is a social, rather than solitary event."

"We suggest that shoulder size may have been an important constraint during hominid birth and, along with increased neonatal cranial size, may have been among the causes of natural selection for the more rounded pelvis of later hominids." 1204.


Timing of birth

Smith, R. (1999). The timing of birth. Scientific American, 280(March), 68-75.

Timing of delivery
"Delivery is triggered by the rate of a protein released from the placenta. (unlike non-primate mammals that have a maternal hypothalamic brain source of CRH.) This protein, corticotropin-releasing hormone (CRH) gets into the maternal and fetal circulation and serves as a "clock" predicting delivery as early as 16 weeks (Smith, 1999). Those with higher early rates delivery earlier. Selection on this factor possibly enabled evolution of increasingly larger brain by means of increasingly earlier, i.e. "premature", birth. Other factors in early parturition include other maternal and fetal hormones, large fetal size and poor maternal nutrition.
The same chemistry serves to regulate toad development and is therefor likely to be an old method of controlling developmental processes."

bipedalism and pregnancy

Whitcome, K. K., Shapiro , L. J., & Lieberman, D. E. (2007). Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature, 450, 1075-1080.

As predicted by Darwin, bipedal posture and locomotion are key
distinguishing features of the earliest known hominins.
Hominin axial skeletons show many derived adaptations for
bipedalism, including an elongated lumbar region, both in the
number of vertebrae and their lengths, as well as a marked pos-
terior concavity of wedged lumbar vertebrae, known as a
lordosis4–6. The lordosis stabilizes the upper body over the lower
limbs in bipeds by positioning the trunk’s centre of mass (COM)
above the hips. However, bipedalism poses a unique challenge to
pregnant females because the changing body shape and the extra
mass associated with pregnancy shift the trunk’s COM anterior to
the hips. Here we show that human females have evolved a derived
curvature and reinforcement of the lumbar vertebrae to compen-
sate for this bipedal obstetric load. Similarly dimorphic morphol-
ogies in fossil vertebrae of Australopithecus suggest that this
adaptation to fetal load preceded the evolution of Homo.
Until recently, hominin females spent most of their adult lives
either pregnant or lactating7. Pregnancy augments the mass of the
human female abdomen by as much as 31% (6.8 kg)8, translating the
position of the maternal COM forward and increasing the torque
exerted by the upper body around the hip joints. Although this shift
in mass does not disrupt postural stability in quadrupeds (Fig. 1a, b),
it uniquely destabilizes bipeds whose supporting joints and two-
footed support base lie solely under the hips (Fig. 1c, d). Such gravid
instability can be counteracted by muscles, but sustained recruitment
risks muscle fatigue and increases the likelihood of spinal injury9.