Because our brains are so large, our newborns have unusually big heads. Complicating matters, shape and orientation of the pelvis have been greatly modified because we walk upright on two legs. Uniquely among primates, a human baby must undergo complex rotation to pass through the obstacle course of a woman’s pelvic canal during birth. In fact, astute analyses of brain size and pelvic anatomy in our fossil predecessors by Boston anthropologist Jeremy DeSilva have confirmed the inference that births first began to become challenging when the genus Homo emerged around two million years ago. Indeed, it seems possible that even the earlier australopithecines may have had more difficult births than modern chimpanzees and possibly needed assistance. Despite competing claims, it seems that midwifery — at least in a simple form — may be the oldest profession.
Big heads on big bodies
The human birth canal is tortuous because the inlet into the pelvis is widest from side to side while the outlet is largest from front to back. In 1992, anthropologist Karen Rosenberg graphically described the special two-stage turning sequence needed for a successful birth. When entering the inlet, the infant’s head is usually already rotated with its long axis oriented from side to side rather than from back to front as in nonhuman primates. Then, while passing through the pelvis, the baby’s head is rotated once more to fit the front-to-back orientation of the long axis of the outlet. So its face usually points towards the mother’s back on emergence. Nonhuman primates generally lack such rotation and the newborn’s face is typically directed forward.
Aside from the newborn’s large head, its large body also make human birth tricky. Newborn chimpanzees, gorillas and orangutans weigh roughly four pounds, but human babies are almost twice as heavy at birth — seven pounds on average. So a newborn human’s shoulders are also wide compared to the birth canal and additional juggling is needed for them to squeeze through. Jamming (dystocia) occurs in about one in a hundred human births. In fact, increased birth weight accompanies the current obesity epidemic in industrialized nations and the incidence of shoulder dystocia is rising.
The special features of human birth make it a drawn-out, difficult process, which doubtless explains the term “labour”. In 1999, gynaecologist Leah Albers published results from a cross-cultural study of labour duration for over 2,500 full-term natural hospital births to low-risk mothers supervised by nurse-midwives. On average, birth took almost nine hours for first-time mothers but only about six hours for mothers with previous births. In extreme cases, birth took as long as twenty hours. By contrast, as anthropologist Wenda Trevathan noted in her 1987 book Human Birth, birth is relatively rapid and straightforward in nonhuman primates, which typically give birth in a couple of hours or less.
Midwives as companions
Labour generally lasts longer with first births. It is also widely accepted that stress can increase its duration. A particularly interesting study was published in 2000 by researcher Marion Heres and colleagues. They compared birth outcomes for 60,000 women tended by midwives or general practitioners and for 30,000 women supervised by hospital obstetricians.
In the Netherlands, midwives are only authorized to deliver full-term single infants with normal presentation and are not permitted to administer the birth-inducing hormone oxytocin. If medical problems are identified during pregnancy or labour, obstetricians supervise births in hospital. Heres and colleagues found clear differences between their two groups. In women tended by midwives, births took about six hours for women with previous births and almost four hours longer for first-time mothers, matching the timing Albers reported for women supervised by nurse-midwives. Strikingly, in the Dutch study births took distinctly longer with women supervised by obstetricians: Births to women with previous births were delayed by three-and-a-half hours and those to first-time mothers by over five-and-a-half hours.
You might think that longer-lasting labour under supervision of obstetricians was simply due to hospital confinement. But Heres and colleagues reported that birth duration did not differ between uncomplicated births supervised by midwives in hospital and home births with midwives. So having a familiar, supportive companion present during labour, providing reassurance and encouragement, has demonstrable benefits, doubtless reducing stress.
Midwives to the Rescue
Apart from providing support, trained midwives can also intervene to tackle specific obstetric problems that arise during the final phase and make birth complex and risky. For example, in about a third of cases rotation wraps the umbilical cord around the baby’s neck. As a rule, this is not life-threatening, but occasionally the cord tightly constricts the baby’s neck. If corrective action is not taken quickly, the infant can be strangled.
Additionally, in her 1993 book Birth in Four Cultures, anthropologist Brigitte Jordan reported on studies of Maya childbirth in a Yucatan community in Mexico. If the fetus was wrongly positioned, for example in a breech presentation, the midwife performed manipulation (version) to correct its orientation. Until the 1950s, versions were also widely practiced in the USA. But then a shift to Caesarian sections began, and manipulation is now extremely rare. This is one reason why C-sections have become alarmingly common in the USA (see my February 2014 blog post Deliver Unto Caesar: A Surgical Epidemic).
In any event, the biblical connection between Eve eating forbidden fruit from the Tree of Knowledge and painful childbirth is intriguing in view of the clear link between large brain size and challenging human births. And midwives can certainly make things easier.
Albers, L.L. (1999) The duration of labor in healthy women. J. Perinatol. 19:114-119.
DeSilva, J.M. (2011) A shift toward birthing relatively large infants early in human evolution. Proc. Natl. Acad. Sci. U.S.A. 108:1022-1027.
DeSilva, J.M. & Lesnik, J.J. (2008) Brain size at birth throughout human evolution: A new method for estimating neonatal brain size in hominins. J. Hum. Evol. 55:1064-1074.
Heres, M.H.G., Pel, M., Borkent-Polet, M., Treffers, P.E. & Mirmiran, M. (2000) The hour of birth: comparisons of circadian pattern between women cared for by midwives and obstetricians. Midwifery 16:173-176
Jordan, B. (1993) Birth in Four Cultures: A Crosscultural Investigation of Childbirth in Yucatan, Holland, Sweden, and the United States (Fourth Edition). Prospect Heights, IL: Waveland Press.
Rosenberg, K.R. (1992) The evolution of modern human childbirth. Yrbk. Phys. Anthropol. 35:89-124.
Rosenberg, K.R. & Trevathan, W. (1996) Bipedalism and human birth: The obstetrical dilemma revisited. Evol. Anthropol. 4:161-168.
Rosenberg, K.R. & Trevathan, W.R. (2001) The evolution of human birth. Sci. Am. 285(5):72-77.
Trevathan, W.R. (1987). Human Birth: An Evolutionary Perspective. Hawthorne, NY: Aldine de Gruyter.