Life On the Outside
A few days ago, his brain's hypothalamus sent a message to his pituitary gland. His pituitary, in turn, sent a signal (a small dose of the chemical adrenocorticotropin, or ACTH) to his adrenal glands, which subsequently played their part by releasing a stress hormone, called cortisol, through the umbilical cord to the placenta. There, it has begun to promote the production of estrogen, suppressing the manufacture of progesterone and, at the same time, releasing enzymes that are capable of converting progesterone into estrogen. Thus, without raising the suspicions of his mother, the fetus -- as seemingly helpless as he is -- has triggered the processes of his birth.
All throughout her pregnancy, his mother's abdominal muscles have been held in stasis by the predominance of progesterone over estrogen. Enough progesterone has kept her body from rejecting the foreign object of her fetus; the right amount of estrogen, on the other hand, has made mild contractions possible, keeping the abdominal muscle toned.
Now that contractions are required to bring the pregnancy to a close, the hormonal ratio must begin to shift in favor of estrogen. The softening and dilation of the cervix is reliant on estrogen, as is the production of proteins that encourage contractions. The ability of the mother's blood to coagulate during the birthing process depends on estrogen; so does the production of prostaglandin and oxytocin receptors. Without these, vaginal childbirth would be impossible, for it's only after estrogen makes the mother's uterus more suspectible to oxytocin -- and oxytocin and prostaglandin then stimulate the contractions of smooth muscles in the uterine wall -- that the whole choreography of labor can genuinely begin.
To get the contractions moving at some meaningful tempo, what's needed is a dose of oxytocin, whose production is a collaborative effort: it's released by the pituitary glands of both the mother and the fetus. The release of the oxytocin promotes further hormonal releases from the adrenal and pituitary glands of the fetus. Mother and child have officially entered the first stage of labor.
Contractions will start coming every 15 to 20 minutes or so now, each lasting 30 to 60 seconds. For the mother, there's pressure and pain. For the fetus, there's the sensation of being squeezed. He hears voices outside himself, feels the rock and walk of his mother's pelvis. As labor progresses and the cervix starts dilating, contractions come more forcefully, more quickly, bring more pain to the mother. With his own head, the fetus helps the process along -- stretching his mother's cervix until it initiates a new cascade of nerve impulses that travel up the mother's spine to her pituitary gland, which answers with the release of more oxytocin. This further stimulates the contractions. Inside, the fetus feels the squeeze and release of the muscular chamber that's housed him these many months.
With every contraction, the uterus shortens and closes, inching the fetus farther down. With every contraction, the cervix widens. If things seemed tight before, they're nothing short of crowded now. The contractions are coming far more often, and the fetus is dimly aware of an array of new sounds outside the womb. He hears the groan of his mother, the soothing voice of his father. Now he is squeezed tightly; it's not pain he feels, but pressure. The uterine muscles clamp down on his head with such persuasion that he begins to release the thyroid hormones and adrenaline that his body instinctively knows it will need to adjust to the cooler temperatures in the outside world.
The pressure from the contractions, as well as the rising concentration of prostaglandins, also protect him from a serious danger: the temptation to breathe. Breathing before his head is completely free of the birth canal would mean taking in fluids and risking pneumonia, so the fetus obeys the blueprint in his genomes that instruct him to wait until the world can hear him cry. It might sound easy, but his oxygen supply is getting thin. His umbilical cord, with its many vessels supplying oxygen-rich blood, is stretching. His mother's blood vessels are squeezed. The placenta is starting to tear away from the anchor of her uterus. But he mustn't yet breathe.