A normal menstrual cycle lasts twenty-eight days. The first day a woman starts her period is day one of her menstrual cycle. During the first fourteen days of the menstrual cycle the ovaries make increasing amounts of the estrogens. The function of these hormones is to stimulate the growth of the endometrial lining, the tissue that covers the inner surface of the uterus. This two-week period during which estrogen hormones are highest is termed the proliferative stage.
Midway through a woman’s cycle, around day fourteen, one of her two ovaries will produce an egg. This is called ovulation. After ovulation, the ruptured follicle from which the egg has been released is transformed to a corpus luteum and begins producing progesterone as well as a small amount of testosterone. Both progesterone and testosterone, which peak just after ovulation, stimulate a woman’s desire for sexual relations.
The portion of the menstrual cycle that follows ovulation, called the secretory phase, is orchestrated by progesterone. Progesterone’s primary function is to mature the endometrial lining, preparing it for a potential pregnancy. Progesterone’s importance to pregnancy is suggested by its name, which literally means “promoting gestation.” If the egg fails to be fertilized and no pregnancy occurs, the production of both progesterone and the estrogen hormones dramatically falls at the end of the twenty-eight-day menstrual cycle. The endometrial lining is sloughed, leading to a period.
This cycle repeats itself over and over again during a woman’s menstrual life, which extends from the time her periods begin at puberty until her periods cease at menopause.