Pregnancy places unique demands upon the human female body. Hormones are the chemicals which the body produces to facilitate the changes needed to accomodate the growing fetus, stimulate labor, and prepare for lactation. The body even produces a new organ (the placenta) to help produce some of these hormones.
Your menstrual cycle actually was the beginning of this process of change. Even before ovulation and fertilization, the cells lining the Dominant Follicle (the egg’s home in the ovary) began producing hormones. These hormones were preparing the lining of the uterus to accept the fertilized egg. But these hormones, progesterone and estrogen in particular, were circulating to other parts of the body as well (your breasts, brain, blood vessels and bone, for example). Soon after ovulation, the now-ruptured Dominant Follicle transformed itself into a progesterone factory called the corpus luteum (or “yellow body”, due to the presence of fat which was being use as raw materials for progesterone production).
It is felt that the progesterone produced in the last half of the menstrual cycle (the “luteal phase”, named after the corpus luteum) is primarily responsible for maintenance of the uterine cell lining (the “endometrium”) for implantation by the fertilized egg. The very roots of the word, progesterone, mean “for (‘pro’) pregnancy (‘gest’)”. In fact, in some cases a woman with a history of repeated miscarriages may be given pharmaceutical progesterone early in pregnancy in an effort to help maintain the early pregnancy…..the theory being that she may have a malfunctioning corpus luteum. Progesterone has other “target tissues” though, too. The breasts respond to progesterone. Progesterone is probably responsible, at least in part, for the breast swelling and tenderness that most pregnant women experience. Later in pregnancy, progesterone produced by the placenta will finish the job of preparing the breasts for lactation…….after all, we’re mammals.
Soon after implantation of the fertilized egg in the endometrium of the uterus, the microscopic pregnancy begins to produce some hormones of its own. In addition to functioning as the fetus’s lungs and source of nutritional transfer, the early cells of the placenta become a hormone factory. Its cells multiply and differentiate (become specialized). These cells produce several hormones which support both the growth of the fetus and the now-pregnant maternal body. Although progesterone usually gets top billing in this show, there are several other hormones produced during pregnancy which are important.