Labor for first time moms averages somewhere between 10 – 20 hours. Among other Great Apes, such as Chimpanzees, labor takes on average two hours. In addition to the time in labor, human labor is described as extremely difficult compared to other primates.
What characteristics contribute to such disparate childbirths in humans and other primates?
1. The Obstetric Dilemma
The most significant tradeoffs in human evolution are evidenced at the time of birth: efficient bipedal locomotion promotes a relatively narrow pelvis, while delivering relatively large-brained babies selects for a broad pelvis. In other words, the narrow pelvic opening that is good for efficient upright walking is not so good for the delivery of large-brained humans.
The interaction of these features in our evolutionary history has shaped a female pelvis that reflects a compromise in these selective pressures: the constraints on the human pelvis due to walking on two legs, combined with the exceptionally large brains of human babies make human birth especially long and painful.
2. Bipedalism Promotes a Narrow Pelvis
Humans are the only living mammal to walk on two legs, and bipedalism requires narrow, short, squat, and basin shaped pelvic anatomies. In contrast, quadrupedal animals (like our closest relatives, chimpanzees) have pelvises that are long and two dimensional in appearance. Compare image A (the pelvis of a chimpanzee) to image C (the pelvis of a human female).
Comparison of pelves from (A) Pan, (B) Australopithecus, (C) Human female, and (D) Human male (from Lovejoy, 2005, figure 5).
Notice how long the chimpanzee (image A) pelvis is compared to the squat pelvis of a human female (image C)? That humans walk on two legs is one explanation for the pronounced differences between humans and chimpanzees.
3. Big Brains and Birth
One important characteristic of primates as a group is a high encephalization quotient, or a large head and brain relative to body size. For most primates, this means that neonates at birth have heads that are close to the size of the maternal birth canal through which they must pass, making birth far from easy.
From Rosenberg and Trevathan (2002); based on Shultz (1949).
The image above describes the relationship of the maternal pelvic inlet (outer white ovals) and size of the baby’s head (dark solid ovals).
Think of a hard boiled egg. For gorillas and chimpanzees, the baby’s head is the size of the yolk, while the mother’s pelvic opening is the size of the egg white. Though baby must pass through the muscles and tissue of the birth canal, there is no bony resistance from the mother’s pelvis. For humans, imagine the hard boiled egg as the shape of the pelvic opening, and a golf ball as the baby’s head.
4. Passage through the Birth Canal
It’s not just the tight squeeze that makes birth difficult – the human birth canal isn’t a constant shape in cross-section. Thanks to bipedalism, the the birth canal is twisted in the middle, while the birth canal is described as a “relatively straight tube” for quadrupedal primates. A monkey baby has a tight squeeze through his mother’s pelvis, however her birth is significantly less difficult than a human baby’s. This is because, unlike the complicated cross-sectional area of the human birth canal, a monkey’s birth canal maintains the same cross-sectional dimensions from front to back.
Image by Karen R. Rosenberg and Wenda R. Trevathan
As a human baby makes her way down the birth canal, she enters the birth canal at it’s widest side-to-side, relative to the mother’s body. But midway through, the orientation of the birth canal shifts 90 degrees, and the baby must now navigate a series of twists and turns so that her largest dimensions – the head and shoulders – are aligned with the largest areas of the birth canal.
The baby must rotate through the changing dimensions of the birth canal twice in order to navigate and pass through the differing pelvic dimensions through the birth canal’s entrance and exit. Our unique mechanism of birth is one of the distinguishing features of human birth, and adds an additional source of difficulty for so many mothers and babies.
5. Differing Degrees of Cervical Dilation
Among primates, human babies have exceptionally big brains. Because of this, the female cervix must dilate three times as much as other primates in order to deliver the big-brained, broad shouldered baby.
Chimpanzee mothers must dilate around 3.3 centimeters before they’re able to give birth. Human mothers must dilate 10 centimeters before they’re able to deliver. The extra 6.6 centimeters of dilation takes significantly more time, but later stages of dilation are also experienced as significantly more painful. As described in the Mayo Clinic, the early stages of labor (0 – approximately 4cm) aren’t always experienced as particularly uncomfortable, and certainly don’t require the same degree of attention as later stages of labor. Later stages of dilation (around 8, 9, 10 centimeters) leading up to transition and delivery are when women experience the most pain.
In Sum: the Obstetric Dilemma
Researchers eloquently describe the interaction of demands as a “tug of war” exerted on the maternal pelvis, which makes human birth significantly more risky than birth in other primates and mammals. Others have described this tradeoff as a scar of human evolutionary history, or the obstetric dilemma.
The Good News: Delivering with Company
Unlike most primates who seek solitude during parturition, virtually all women cross-culturally seek out assistance during birth from relatives, doulas, obstetricians, midwives, and other supports. While there are stories of women who had successful solitary births, studies of childbirth across cultures find that these stories are exceptional. Across cultures, women seek out both physical and emotional assistance in labor and delivery.
If you’re pregnant, consider giving birth with continuous emotional support: from a partner, family member, friend, or doula. A woman’s desire to be surrounded by familiar “others” has deep roots in our human lineage, is associated with fewer birthing complications.
Please feel free to contact me at firstname.lastname@example.org, or in the form below:
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