Sunday, October 9, 2011 - 0 comments

The Evolution of Human Birth : 2. Tight Squeeze

By : Karen R. Rosenberg and Wenda R. Trevathan

TO TEST OUR THEORY that the practicetice of assisted birth may have been around for millennia, we considered first what scientists know about the way a primate baby fits through the mother’s birth canal. Viewed from above, the infant’s head is basically an oval, longest from the forehead to the back of the
head and narrowest from ear to ear. Conveniently, the birth canal—the bony opening in the pelvis through which the baby must travel to get from the uterus to the outside world—is also an oval shape. The challenge of birth for many primates is that the size of the infant’s head is close to the size of that opening.

For humans, this tight squeeze is complicated by the birth canal’s not being a constant shape in cross section. The entrance of the birth canal, where the baby begins its journey, is widest from side to side relative to the mother’s body. Midway through, however, this orientation shifts 90 degrees, and the long axis of the oval extends from the front of the mother’s body to her back. This means that the human infant must negotiate a
series of turns as it works its way through the birth canal so that the two parts of its body with the largest dimensions—the head and the shoulders—are always aligned with the largest dimension of the birth canal [see illustration at right].

its head against the mother’s pubic bones,
makes it difficult for a human female to guide the
infant from the birth canal—the opening in the
mother’s pelvis (insets)—without assistance.
To understand the birth process from the mother’s point of view, imagine you are about to give birth. The baby is most likely upside down, facing your side, when its head enters the birth canal.  Midway through the canal, however, it must turn to face your back, and the back of its head is pressed against your pubic bones. At that time, its shoulders are oriented side to side. When the baby exits your body, it is still facing backward,
but it will turn its head slightly to the side. This rotation helps to turn the baby’s shoulders so that they can also fit between your pubic bones and tailbone. To appreciate the close correspondence of the maternal and fetal  dimensions, consider that the average pelvic opening in human females is 13 centimeters at its largest diameter and 10 centimeters at its smallest. The average infant head is 10 centimeters from front to back, and the
shoulders are 12 centimeters across. This journey through a passageway of changing cross-sectional shape makes human birth difficult and risky for the vast majority of mothers and babies.

If we retreat far enough back along the family tree of human ancestors, we would eventually reach a point where birth was not so difficult. Although humans are more closely related to apes genetically, monkeys may present a better model for birth in prehuman primates. One line of reasoning to support this assertion
is as follows: Of the primate fossils discovered from the time before the first known hominids, one possible remote ancestor is Proconsul, a primate fossil dated to about 25 million years ago. This tailless creature  probably looked like an ape, but its skeleton suggests that it moved more like a monkey. Its pelvis, too, was more monkeylike. The heads of modern monkey infants are typically about 98 percent the diameter of the
mother’s birth canal—a situation more comparable with that of humans than that of chimps, whose birth canals are relatively spacious. Despite the monkey infant’s tight squeeze, its entrance into the world is less challenging than that of a human baby. In contrast to the twisted birth canal of modern humans, monkeys’
birth canals maintain the same cross-sectional shape from entrance to exit. The longest diameter of this oval shape is oriented front to back, and the broadest part of the oval is against the mother’sback. A monkey infant enters the birth canal headfirst, with the broad back of its skull against the roomy back of the mother’s pelvis and tailbone. That means the baby monkey emerges from the birth canal face forward—in other words, facing the same direction as the mother.

Firsthand observations of monkey deliveries have revealed a great advantage in babies’ being born facing  forward. Monkeys give birth squatting on their hind legs or crouching on all fours. As the infant is born, the mother reaches down to guide it out of the birth canal and toward her nipples. In many cases, she also wipes mucus from the baby’s mouth and nose to aid its breathing. Infants are strong enough at birth to take
part in their own deliveries. Once their hands are free, they can grab their mother’s body and pull themselves out.

If human babies were also born face forward, their mothers would have a much easier time. Instead the evolutionary modifications of the human pelvis that enabled hominids to walk upright necessitate that most infants exit the birth canal with the back of their heads against the pubic bones, facing in the opposite
direction as the mother (in a position obstetricians call “occiput anterior”). For this reason, it is difficult for the
laboring human mother—whether squatting, sitting, or lying on her back—to reach down and guide the baby as it emerges. This configuration also greatly inhibits the mother’s ability to clear a breathing passage for the infant, to remove the umbilical cord from around its neck or even to lift the baby up to her breast. If she tries to accelerate the delivery by grabbing the baby and guiding it from the birth canal, she risks bending its back awkwardly against the natural curve of its spine. Pulling on a newborn at this angle risks injury to its spinal
cord, nerves and muscles.

For contemporary humans, the response to these challenges is to seek assistance during labor and delivery. Whether a technology-oriented professional, a lay midwife or a family member who is familiar with the birth process, the assistant can help the human mother do all the things the monkey mother does by herself. The assistant can also compensate for the limited motor abilities of the relatively helpless human infant. The advantages of even simple forms of assistance have reduced maternal and infant mortality throughout history.

Next : Assisted Birth


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