Women of childbearing age exhibit rhythmic changes in the production of reproductive hormones. This pattern, commonly referred to as the menstrual cycle, generally averages 28 days. The timing of the cycle is regulated by complex interactions among many hormones and other chemical messengers produced in the brain, reproductive organs, and other tissues. The hormones of primary interest in this article belong to a class of substances called steroids, which are synthesized within the body from dietary cholesterol.
The principal female sex steroids, progestrone and estrogens,(1) are produced largely in the ovaries and uterus. Among other functions, estrogens stimulate the proliferation and growth of cells within the sexual organs. Progesterone primarily facilitates pregnancy and lactation.
The menstrual cycle is considered to begin with menstruation. The midpoint of the cycle is characterized by ovulation, in which a single egg cell (ovum) is expelled from the ovary. If the ovum is fertilized by a sperm, it lodges in the uterus and develops into a fetus.
Phases of the Menstrual Cycle
The menstrual cycle has five phases, as described below.
Menstrual Phase (Days 1-5). Menstrual flow occurs if the expelled ovum has not become fertilized during the ovulatory phase (see below). Levels of progesterone and estrogens are low.
Follicular Phase (Days 6-12). The ovary secretes small but increasing quantities of estrogens, stimulating the development within the uterus of specialized cell clusters called follicles. Each follicle nurtures a developing ovum and further contributes to the secretion of estrogens. Only one follicle reaches full maturity.
Ovulatory Phase (Days 13-15). The mature follicle ruptures, releasing its ovum. Fertilization can occur at this time.
Luteal Phase (Days 16-23). The empty follicle becomes a glandlike structure called the corpus luteum, which secretes large quantities of estrogens and progesterone. These hormones prepare the uterine lining for implantation of the fertilized ovum.
Premenstrual Phase (Days 24-28). If the ovum is not fertilized, the corpus luteum degenerates and stops secreting progesterone and estrogens. Decreased levels of these hormones result in the shedding of the uterine lining (i.e., menstruation). If pregnancy occurs, the corpus luteum remains intact and secretes increasing quantities of steroid hormones, promoting the development of the fetus and the progression of pregnancy.
Summary
Levels of both estrogen and progesterone are low from day 28 to day 7 (i.e., during menstruation) and high between days 19 and 25. During days 12 to 14, estrogen levels are high, whereas progesterone levels are low [ILLUSTRATION FOR FIGURE OMITTED].
- John J. Doria
1 The primary estrogen in humans is known as estradiol.
COPYRIGHT 1999 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group
