The Intricacies of Human Birth: A Natural Phenomenon

Polio is a disease that strikes children almost exclusively–more than 80 percent of paralysis cases occur in children under age five. It is caused by an intestinal virus; the virus must be ingested to bring about an infection. Once inside the gut, it passes through the lining and takes up residence in nearby lymph nodes. There it multiplies, produces fevers and stomach upset, and passes back into the feces. Those infected can contaminate their clothing, bathing sites, and supplies of drinking water and thereby spread the disease. (The virus can survive as long as sixty days outside the body.)

Poliovirus infects only a few kinds of nerve cells, but what it infects it destroys. In the most dreaded cases, the virus spreads from the bloodstream into the neurons of the brain stem, the cells that allow you to breathe and swallow. To stay alive, a person has to be fed through a tube and ventilated by machine.

The nerve cells most commonly attacked, though, are the anterior horn cells of the spinal cord, which control the arms, the legs, and the abdominal muscles. Often, so many neurons are destroyed that muscle function is eliminated altogether.

Human birth is an astonishing natural phenomenon. The process is a solution to an evolutionary problem: how a mammal can walk upright, which requires a small, fixed, bony pelvis, and also possess a large brain, which entails a baby Whose head is too big to fit through that small pelvis. Part of the solution is that, in a sense, all human mothers give birth prematurely. Other mammals are born mature enough to walk and seek food within hours; our newborns are small and helpless for months. Even so, human birth is a feat involving an intricate sequence of events. First, a mother’s pelvis enlarges. Starting in the first trimester, maternal hormones stretch and loosen the joints holding the four bones of the pelvis together. Almost an inch of space is added. Pregnant women sometimes feel the different parts of their pelvis moving when they walk.

Then, when it’s time for delivery, the uterus changes. During gestation, it’s a snug, rounded, hermetically sealed pouch; during labor it takes on the shape of a funnel. And each contraction pushes the baby’s head down through that funnel, into the pelvis. This happens even in paraplegic women; the mother does not have to do anything.

Meanwhile, the cervix-which is, through pregnancy, a rigid, more-than-inch-thick cylinder of muscle and connective tissue capping the end of the funnel–softens and relaxes. Pressure from the baby’s head gradually stretches the tissue until it is paper-thin–a process known as “effacement.” A small circular opening appears, and each contraction widens it, like a tight shirt being pulled over a child’s head. Until the contractions pull the cervix open about four inches, or ten centimeters–the full temple-to-temple diameter of the child’s head–the child cannot get out. So the state of the cervix determines when birth will occur. At two or three centimeters of dilation, a mother is still in “early” labor. Delivery is many hours away. At four to seven centimeters, the contractions grow stronger. “Active” labor has begun. At some point, the amniotic sac surrounding the fetus breaks under the pressure, and the clear fluid gushes out.

Contractile force increases further.

At between seven and ten centimeters of cervical dilation, the “transition phase,” the contractions reach their greatest intensity. The contractions press the baby’s head into the vagina and the narrowest part of the pelvis’s bony ring. The pelvis is usually wider from side to side than front to back, so it’s best if the baby emerges with the temples- the widest portion of the head–lined up side to side with the mother’s pelvis. The top of the head comes into view. The mother has a mounting urge to push. The head comes out, then the shoulders, and suddenly a breathing, wailing child is born. The umbilical cord is cut. The placenta separates from the uterine lining, and with a slight tug on the cord and a push from the mother, it is extruded. The uterus spontaneously contracts into a clenched ball of muscle, closing off its bleeding sinuses-the expanded veins in the uterine wall. Typically, the mother’s breasts immediately let down with colostrum, the first milk, and the newborn can latch on to feed.

That’s if all goes well. At almost any step, the process can go wrong. For thousands of years, childbirth was the most common cause of death for young women and infants. There’s the risk of hemorrhage. The placenta can tear or separate, or a portion may remain stuck in the uterus after delivery and then bleed torrentially. Or the uterus may not contract after delivery, so that the raw surfaces and sinuses keep bleeding until the mother dies of blood loss. Sometimes the uterus ruptures during labor.

The most basic problem is “obstruction of labor”-not being able to get the baby out. The baby may be too big, especially when pregnancy continues beyond the fortieth week. The mother’s pelvis may be too small, as was frequently the case when lack of vitamin D and calcium made rickets common.

The baby might arrive at the birth canal sideways, with nothing but an arm sticking out. It could be a breech, coming butt first and getting stuck with its legs up on its chest. It could be a footling breech, coming feet first but then getting wedged at the chest with the arms above the head. It could come out headfirst but get stuck because its head is turned the wrong way. Sometimes the head makes it out, but the shoulders get stuck behind the pubic bone of the mother’s pelvis.

Source : Better: A Surgeon’s Notes on Performance by Atul Gawande

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