Before discussing delivery, we must first consider the forces involved in birth.
It is necessary to imagine the baby confined in a large, gourd-shaped, elastic bottle, the muscular uterus. This lies within the mother’s abdomen, the bottom of the bottle under her ribs, and the mouth deep in her pelvis. The neck of the bottle, the cervix (Latin: ‘neck’), is about a half-inch long when labor begins, and it is almost tightly closed. Before a full-term baby can be expelled from the bottle, the neck must be stretched to a diameter of four inches (ten centimeters, the minimum necessary to allow the baby to pass from the uterus into the narrow, stiff-walled, five-inch corridor (pelvis and birth canal) which leads to the outside world.
The motive power that dilates the mouth of the bottle and propels the child through the resistant birth corridor is mainly the force generated by the contractions of this large, muscular organ. This largest muscle of the body, far heavier and greater than the powerful biceps of a heavyweight champion prizefighter, forms a complete elastic casing about the child except for the small opening at its neck—the relatively weak point in the wall. When the muscle fibers contract, the pressure within the uterus is greatly increased—the process is like compressing a water-filled rubber bulb—and this increased pressure is transmitted to the constraining walls. Many computations have been made of the force of a single labor pain, and figures varying from four to one hundred pounds have been obtained by separate investigators. From their data, an average of between twenty-five and thirty pounds seems likely. This force is directed against the cervix in two ways. The uterus has a completely separate inner fining, the membranes which contain the child and the amniotic fluid. During a pain both the baby and the fluid surrounding it are put under thirty pounds of pressure in all directions. When the membranes are intact, this force drives the fluid against the cervix; when they are ruptured, the baby is driven against it. Through pressure exerted by either the fluid or the baby, the cervix is forced open, stretched open from within. In addition to this pressure from within (dilatation), the neck of the bottle is shortened and pulled open by the contraction of the muscle fibers of the lower portion of the uterus, to which the muscle fibers of the cervix are attached (retraction). About nine of the ten and one-half hours of a primiparous labor and five and three-quarters of the six and one-half hours of a muciparous labor are occupied in stretching the opening in the cervix to its maximum four-inch diameter. This phase of labor is called the first stage. When the cervix is wholly dilated, the baby no longer meets resistance in the upper birth canal, and the full force of the uterine contractions is spent in driving it through the lower birth canal. In this stage of labor (the second stage) the force of the uterine contractions is greatly augmented by involuntary contractions of the woman’s abdominal muscles, to which are added her own bearing down efforts. Instinctively she fills her lungs with air, fixes her diaphragm by closing her epiglottis, and increases her intra-abdominal pressure by straining as though at stool. These activities increase the uterine pressure to about sixty pounds. The primiparous woman expels her infant after an hour and a half of the second stage; the multipara requires a second stage only half as long. The third stage, the time elapsing from the birth of the baby to the delivery of the placenta, lasts but a few minutes.