fertility, ability of an individual or couple to reproduce through normal sexual activity. About 90 percent of healthy, fertile women are able to conceive within one year if they have intercourse regularly without contraception. Normal fertility requires the production of enough healthy sperm by the male and viable eggs by the female, successful passage of the sperm through open ducts from the male testes to the female fallopian tubes, penetration of a healthy egg, and implantation of the fertilized egg in the lining of the uterus (see reproductive system). A problem with any of these steps can cause infertility.

menstrual cycle

biology

menstrual cycle, recurring fluctuations in hormone levels that produce physical changes in the uterus and ovaries to prepare the female body for pregnancy. In adult women, the menstrual cycle lasts anywhere from 21 to 40 days, with the average being 28 days. The cycle recurs until menopause, or roughly about age 50, at which time ovarian function ceases, bringing an end to the woman’s reproductive life. The menstrual cycle is marked by two events: ovulation and menstruation.

Phases of the menstrual cycle

The normal human menstrual cycle is 28 days, but no woman is always precisely regular, and cycles as short as 21 days or as long as 40 days are not abnormal. It is customary to call the first day of the menstrual period the first day of the cycle, although menstruation is the end rather than the beginning of a process. On this basis the cycle is described as starting with about five days of menstruation, followed by a proliferative phase that lasts to about the 14th day, and then a secretory phase that lasts until the next menstruation. The external manifestation of menstruation depends upon cyclical change in the lining of the body of the uterus. The lining, called endometrium, consists of tubular glands that open into the uterine cavity. The glands lie in a vascular framework, or stroma, and are separated by it.



At the end of menstruation, just at the beginning of the proliferative phase, the endometrium is thin, with short, straight glands, and the ovary is quiescent. Under the influence of the gonadotropic hormones from the pituitary gland, an ovarian follicle (occasionally more than one) ripens in one of the ovaries. This ovarian follicle contains the ovum, which is a cell about 0.14 millimetre (0.006 inch) in diameter, surrounded by a group of smaller cells, called granulosa cells. The granulosa cells multiply, with the ovum situated in the wall of the rounded structure that they form, and secrete an estrogenic hormone, estradiol (see hormone). This hormone causes proliferative changes in the endometrium, so that the glands become taller and the whole endometrium becomes thicker and more vascular.

Hormonal control of menstrual cycle

The ovarian hormones circulate in the blood and are excreted in modified forms in the urine. Estimation of the urinary output by chemical methods gives an indication of the blood levels and of the total production of these substances. There are several natural estrogens, and numerous synthetic modifications of these and of progesterone have been devised; many are active when taken by mouth and are used for treatment of hormonal disorders and as oral contraceptives.

Ovulation and the fertile phase

Ovulation occurs at about the midpoint of each normal cycle, and the ovum is probably capable of fertilization for only about two days after this. In the majority of women the time of ovulation is fairly constant. In women with cycles of irregular length the date of ovulation is uncertain; in these women the long menstrual cycles are usually due to prolongation of the proliferative phase; the secretory phase tends to remain normal in length. In some animals, ovulation only follows coitus; this mechanism has been used to explain cases in which human pregnancy has apparently followed coitus early or late in the menstrual cycle, but there is no definite evidence for such a mechanism in women.

The rhythm method of contraception is based on the fact that ovulation normally occurs at mid-cycle, but the date of ovulation may vary unexpectedly even in women whose menstrual cycles were previously regular.