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Mammals Giving Birth

Mammals, Birth, and Movement

I have always been fond of telling my students on the first day of class to start thinking of themselves as MAMMALS. In preparing to give birth, I believe the concept that they are merely human mammals helps them recognize that our birthing is as primal as that of all mammals.

However, I also found this information from BIRTH DAY: A Pediatrician Explores the Science, the History, and the Wonder of Childbirth by Mark Sloan, M.D. enlightening, and I wish to alter my analogy. I don’t think it means we have to view human birth as necessarily complicated or dangerous; I do think it means we have to STRONGLY BRING TO OUR AWARENESS the need for laboring women to move freely, change positions, and be accorded freedom, patience and privacy, and to consider in more depth why epidurals and lack of mobility impede the labor process.


A female gorilla wanders off from the group, has her baby in about half an hour, ignored by the others, and with few visible signs of discomfort. Human birth, especially a first baby, is commonly about 18 hours, and usually requires assistance.

In the anatomy of the gorilla birth, the female pelvis is roomy, the fetus small, birth is a straightforward process. A gorilla fetus starts and finishes labor in the same position - head down, facing forward toward the mom’s abdominal wall. No twists or turns, no shifting in negotiating its exit. Just down, down, and out.

Human birth is more like a bobsled run. It has its start and finish points, but in between are banked curves and turns that the fetus steers.

The gorilla pelvis/birth canal is like a compressed cylinder, like a cardboard toilet paper roll with its sides gently squeezed. From top to bottom, the roll is uniformly widest in the front-to-back dimension.

In the human female pelvis, the upper third is widest in the side-to-side dimension, while the lower two-thirds is widest from front to back, like the gorilla’s. Take the gorilla’s gently squeezed toilet paper roll, and over much time (evolution), pinch the upper third sideways, so that the top and bottom are now perpendicular to one another. The human fetus must navigate this winding road.

The human fetus needs time, patience, and MOVEMENT of the mom, to negotiate its path.

A typical human fetus in the 3rd trimester is head down, chin tucked on chest, arms & legs crossed. The gorilla fetus faces forward; the human faces backward, toward the mom’s spine. The gorilla drops straight onto the soft tissues; the human turns its head to the side, as though checking over the shoulder. This sideways-facing head then enters the pelvic inlet (opening of the bony pelvis) and begins its descent. On its way to being born, the fetus flexes, extends, and rotates its body, executing a series of maneuvers not seen in any other large primate.

Why is human birth not the straightforward route of other large primates?

Several things happened, over millions of years, as humans evolved from the ‘big enough’ maternal pelvis and the small-brained fetus.

Change began when we came out of the trees and became vertical. Bipedal locomotion allowed prehumans to see over the tall savannah grasses (central Africa) and to run from predators, along with freeing hands for tools and weapons. Over time, the natural selection for upright walking changed the shape of the human skeleton, especially the pelvis. Lucy’s pelvis (3 million years ago) was already wider than an ape’s and her inlet had already taken on the oval, sideways-oriented shape of a modern female. Because of the wider, oval-shaped inlet, a fetus had to turn its head to the side when it entered the birth canal. The maternal pelvis could only grow so wide to accommodate a round fetal head, otherwise, the resulting awkward wide gait would make it impossible to flee predators.

Later, as evolution favored larger brains (smarter for making, using tools, etc.), the human brain became bigger. It took thousands of years before the fetal head got large enough to be a problem, bringing us to about 1.5 million years ago. Nature had to either make the container bigger or the contents smaller. Once again, human anatomy and physiology had to adapt.

The bones of the pelvis had accommodated as much as they could. The uterus can expand almost infinitely (think of multiples) but the pelvis can’t. The hormone RELAXIN, produced by the ovaries, skyrockets in pregnancy. The pelvic bones stay rigid but the ligaments connecting them relax and soften. In labor, the fetus pushes against bones, and they give way (especially around the pubic bones & sacroiliac joints). As laboring women move, bend, squat, and stretch, the pelvis changes shape and makes more room.

A fetal head is not like a rigid adult skull - it is a set of loosely joined springy plates. The bones are not fused. The large (diamond-shaped) fontanel is on top/toward the front, and the smaller (triangle-shaped) fontanel is toward the back. The baby’s head presses repeatedly against the cervix, causing dilation, but as the opening to the birth canal gets larger, the baby’s head actually gets longer and thinner. The bony plates crowd together, then slightly slide over each other (what we see as a “conehead”). This change in shape decreases the width of the head by as much as half an inch, which can be a lot for a 3-4 inch wide exit. The brain is like thick pudding, changing shape along with the skull. (After vaginal birth, you can feel a hard ridge from front to back of baby’s head where the plates lie on top of each other like a continental collision. Within days the plates slide back into place.)

The gorilla baby’s skull has no soft spot, ridge, or coning. Their skull plates are fused before birth and cannot slide over each other, but still easily pass through the spacious birth canal.

The shoulders are an inch or so wider than the head. The collarbones are springy, like the skull. They bend, twist, and compress; the ridges can even slide over each other like the skull plates, making the breadth narrower. Broad shoulders may be extracted one at a time. Attendants may need to pull downward to release the upper shoulder, then upward to release the lower shoulder - clearly less trauma than if both shoulders emerged simultaneously.

The human skull with its soft spots and non-fused bony plates is the first key difference between human and ape newborns. The second is the complete dependency of human newborns on adult care. Both are part of the same childbirth compromise.

The female pelvis, after radically redesigning itself to accommodate upright walking, and the fetal head, after adapting itself to fit through the more complicated passage, had done all the compromising they could. To insure that the fetal head size remained more or less constant, our ancestors then birthed their babies in earlier and earlier stages of development. This allowed for relatively safe passage down the birth canal but resulted in totally vulnerable newborns. Neurological and physical growth that once happened in utero now takes place after birth. Our babies are born helpless and require the longest amount of parental care than any mammalian species. They are actually premature in comparison to non-human mammals. A newborn horse can run, a dolphin can swim, a gorilla cling.

Laboring women prefer sitting, standing, squatting, kneeling, and changing positions. They use posts, ropes, knotted cloths, hammocks, bricks, stones, birthing chairs or stools, and companions to find comfortable positions. None voluntarily lie on their back (the original motive for lying the woman flat came from Louis XIV’s wish to view the birth of his child in the late 1600s). Studies show that the positions they choose can enlarge their pelvic outlets by as much as 30%. Women who are free to move about have easier births; they are maximizing the available space for the baby to pass through.

The human fetus needs time, patience, and MOVEMENT of the mom, to negotiate its path. 

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