Although it is possible for human females to deliver their infants without assistance, mortality and morbidity are significantly reduced with the help of others. Worldwide it is far more common for birth to occur in the presence of others than alone and even in cultures where solitary births are desirable, first births rarely occur alone. On the other hand, almost all ape births are solitary and the presence of conspecifics is rarely reported in the few births that have been described for non-captive animals. There are a number of reasons for this difference in birth-related behavior in human and apes but the most likely reason stems from the delivery process itself. In humans, the infant is delivered facing the opposite direction from the mother in a position described as “occiput anterior” in the medical literature (that is, the back of the baby’s head, the occiput, emerges from the birth canal against the front, anterior, of the mother’s pelvis). This emergence pattern makes it very difficult for her to wipe birth fluids from the infant’s mouth, to check for the umbilical cord that may be around its neck, and to assist her infant in exiting the birth canal without risking damage to the neck. In contrast, ape infants are typically born facing the mother’s stomach, which allows the mother to reach down and guide the infant. The ape infant can even assist itself by grasping the mother’s hair.
The differing emergence patterns for humans and apes are the result of different modes of locomotion and the skeletal anatomy of bipedalism vs quadrupedalism/knuckle-walking. In apes and monkeys, the pelvic inlet and outlet are both longest in the front-to-back dimension as is the infant’s head. The infant typically enters and exits the birth canal in the same plane, facing forward in the same direction as the mother. In humans, because of adaptation to bipedalism, the pelvic inlet is greatest in the side-to-side dimension whereas the outlet is greatest in the front-to-back dimension. As with apes, the baby’s head is largest in the front-to-back dimension. Because of the large size of the human infant’s head, there is a tight squeeze through the relatively narrow birth canal and all fetal and pelvic dimensions need to line up in order for the baby to pass through without damage. Furthermore, the front of the human pelvis is typically the broadest so the back of the infant’s head aligns with the front of the mother’s pelvis resulting in the infant emerging facing away from the mother.
Unfortunately, there are very few observations of ape births in the wild, and those that do exist do not usually report the emergence pattern of the infant, so it cannot be said with certainty that monkey and ape infants are born facing forward in an occiput posterior position without assistance from others. The relatively spacious birth canals of great apes argues against the inherent difficulty of births, but monkey birth canals are only slightly larger than their neonates’ heads, suggesting that they may benefit from assistance in some circumstances. We need to know more about births in monkeys and apes and we need more detail on the way in which the birth occurs and the context with regard to conspecifics.
Ethnographic reports note the near universality of assistance at birth. Even where solitary births are the ideal, first births often occur with assistance. Surprisingly, there are only a few descriptions of births to humans outside of hospitals or home births in US and Europe that go into enough detail to note the position of the infant’s head as it emerges. There is some suggestion that the facing backward (occiput posterior) position is the norm for births in women who did not engage in hard work and high energy expenditure in childhood when the pelvis was developing and that there is much greater variation in pelvic shapes and sizes than is reported in the medical literature (Walrath, 2003). In other words, there may be some populations in the present and past where the shape of the pelvis resulted in infants being born face forward in which case the mother could more easily complete the delivery without assistance. Analysis of skeletal collections may help reveal how much variation there is the shape and size of the human female pelvis.
The anatomical differences between bipedalism and quadrupedalism account for the differences in the way in which birth occurs in humans and apes. The behavior of seeking assistance may be emotional, however. From an evolutionary medicine perspective, the anxiety that most women feel at the time of birth may actually be a "defense" (Nesse, 1991) rather than the "defect" that it is often seen in clinical pratice. Feelings of anxiety at the time of birth may have led the normally gregarious human female to seek companionship at birth rather than the isolation of her ape ancestors. Those who sought assistance for emotional support probably also received occasional physical support, which may have reduced mortality and morbidity. Natural selection may have thus selected for low levels of anxiety at the time of birth because women who responded by seeking support would have had more surviving offspring than those who delivered their infants alone.
Greater survival for female hominins who sought companionship at birth and greater offspring survival.
When women experience fear and uncertainty in hospital delivery suites in the presence of strangers, they are reflecting the legacy of their ancestors who sought companionship from supportive friends and relatives. In otherwise normal healthy deliveries, providing emotional support by allowing friends, relatives, or doulas in the delivery room may reduce the anxiety, thus contributing to a positive outcome, both medically and emotionally.
There are reports of assisted deliveries in orangutans, baboons, marmosets, tamarins, dolphins, and other mammals (reviewed in Trevathan, 1987) but it is far from a common occurrence.
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Human Birth: An Evolutionary Perspective, , New York, NY, p.286, (1987)