HE FIRST symptom that makes a woman wonder whether she is pregnant is delay in the menstrual period. Since the menstrual cycles of most women do not vary by more than three or four days, delay of a week or Len days suggests pregnancy. If the woman’s periods normally vary by this amount, the delay has no significance. But by the time the second period has been skipped, there is usually very little doubt in her mind or in the ‘ d of her physician. Each month nature has prepared the way for a new pregnancy.
This remarkable cyclic activity in women is under control of the tiny. hormone-producing pituitary gland, at the base of the brain. Hormones are internally secreted chemical substances that stimulate or repress the functioning of specific organs or tissues. The pituitary stimulates the development, or ripening, of eggs in the ovary. At the start of the monthly cycle, which usually has a period of twenty-eight days, some two or three dozen eggs begin to mature.
By about the fourteenth day one egg has matured more than its fellows and is cast forth from the surface of the ovary. This process is called ovulation. The development of the other eggs halts at once. These others regress and are never used again. Immediately after ovulation, the bed or follicle within the ovary where the egg had developed is transformed into a hormone-producing structure. Because the hormones are released directly into the blood-stream from the endocrine glands (ovary. thyroid, adrenal. and others) , the reactions in distant pans of the body begin almost at once. After
ovulation the ovarian follicle assumes a yellow color; it is known as the corpus luteum, from the Latin for “yellow body.” Its product. the hormone progesterone, has diverse effects on the female organism, one of the most important being to arrest the development of the other eggs that had begun to ripen prior to ovulation. If this ripening were not stopped. multiple ovulations and multiple pregnancies would be possible each cycle.
For its ability to inhibit ovulation, progesterone is an ingredient of all birth control pills. Progesterone raises the body temperature. By accurately recording your temperature day after day, you can tell when ovulation and the production of progesterone begin. Probably the most important action of progesterone is to prepare the lining of the womb for pregnancy. The lining must be receptive. If the egg does not become fertilized, the corpus luteum regresses in twelve to fourteen days.
Production of progesterone ceases, and the built-up lining of the womb is cast off as the menstrual discharge. The whole process then starts all over again. For a pregnancy to become established many events must occur at just the right time. The father must be able to supply adequate numbers of sperm cells, and the “adequate” numbers are truly astounding. Normal .males have sperm counts in the neighborhood of fifty to one hundred million for each cubic centimeter of semen and produce two to three cubic centimeters at each ejaculation.
If the sperm count is as low as twenty million for each cubic centimeter, it is difficult for most couples to have children. With counts of five million or less, sterility is usually complete and permanent. Once in the birth canal, the sperm cells have to make their way up through the uterus and Fallopian tubes. We really do not know much about the mechanics of sperm migration and transportation, but time seems to be on their side. The sperm cell apparently is capable of fertilizing the egg even if two or three days have elapsed.
After the journey up the uterus and Fallopian tubes, which lead from the body cavity to the uterus, the sperm has to locate the egg cell and join with it. This union is known as fertilization. Just how the sperm finds the egg is unknown. It is not known even where fertilization occurs. In most animals fertilization appears to occur in the outer portion of the tube. but in human beings there is much evidence that this happens in the general abdominal cavity.
But regardless of where fertilization occurs. the egg must get from the ovary into the tube. While this is a short distance. less than an inch, the egg does not have its own means of locomotion; it must he buffeted or drawn into the tube in some manner
as yet unknown. Pregnancies have occurred in women who by pelvic surgery have lost the ovary on one side and the tube on the other. On these rare occasions, the egg has to migrate several inches to get from the ovary into the tube. While the sperm cells are viable, retaining their ability to fertilize the ovum for two or three days, the mature egg cast off at ovulation must meet and join with the male sperm cell within twelve hours, or twenty-four hours at the most. If not, it will degenerate.
Pregnancy will then not be possible until the following cycle. When you think that there are only a few hours each month in which the female is capable of becoming pregnant, it is truly remarkable how easily most women conceive. The explanation is to be found in the fact that the sperm cells live so long. When a couple is having sexual relations two or three times a week, there are almost always sperm cells around, regardless of when ovulation occurs. After the egg has become fertilized, it splits in half. The two cells so formed in turn divide, giving rise to a four-celled structure.
Cell divisions continue, and the developing ovum assumes a spherical shape. It is wafted down the Fallopian tube by a gentle beating action of tiny hairlike projections on each of the cells lining the tube. The journey down the tube takes three or four days, and after the ovum reaches the cavity of the uterus. it waits another three or four days before becoming attached to the lining of the womb. This step is known as implantation, or nidation, and in humans it occurs roughly a week after ovulation. You will recall that during this week the corpus luteum hormone, progesterone, has been preparing the r g of the womb for implantation.
Once this process has occurred, the developing ovum produces a hormone of its own, called chorionic gonadotropin. Minute amounts of this hormone get into the mother’s bloodstream and arc carried to the ovary, where the hormone signals the corpus luteum to keep up the production of progesterone. If the corpus luteum falters at this time, as it does regularly in cycles where pregnancy does not occur, production of progesterone stops. If this should happen. the entire lining of the womb, including the implanted ovum, would be cast off. One can appreciate how sensitive the whole process is. If events do not occur at just the right moment, it may be difficult or impossible for a woman to conceive.