She knew she couldn't be pregnant. But she had missed three menstrual periods and, even though she felt healthy, she was beginning to wonder if something was wrong. Then she read an article in a nagazine that said amenorrohea (failure to menstruate) was common among female runners. Since she had just taken up jogging six months earlier, she decided this must be the cause of her sudden irregularity. A month later she got her period and thought no more of it.

This jogger may or may not have had the right solution. Menstruation is a finely tuned physiological process, subject to many influences, internal and external. It is not uncommon for a woman occasionally to miss a menstrual period.

It's more unusual and of more concern for a woman to stop menstruating for three months or more, or never to begin at all. This condition is called amenorrhea (a = without; men is from the Greek for month; and rhea = flow, or discharge). A girl who doesn't start menstruating in her teens (usually by 16 to 18, according to most texts) is said to have primary amenorrhea. when menstruation stops after it has begun, the condition is called secondary amenorrhea.

The menstrual cycle involves a number of the body's organs and hormones in a process that stretches the length of the body. The cue to menstruate might be said to start in the brain in an area called the hypothalamus, which at the appropriate time each month releases a hormone called GnRH (which stands for gonadotropin releasing hormone). This causes the pituitary gland, a pea-sized organ attached to the hypothalamus, to release the so-called gonadotropic hormones, LH (luteinizing hormone and FSH (follicle stimulating hormone). (See "The Pituitary: A Kernel Of Wonder" in the February 1984 FDA Consumer).

LH and FSH then stimulate one (or sometimes both) of the ovaries to mature and--in the process called ovulation--produce ova, or eggs, ready to be fertilized. LH and FSH also cause the ovaries to secrete the hormones estrogen and progesterone. The quantity and order in which these hormones are released control the development of the endometrium (the lining of the uterus), which thickens each month to provide a "home" for a fertilized ovum. If the ovum is not fertilized during its journey to the uterus, this build-up of endometrial material is sloughed off and flows out of the body through the cervix and vagina in the process called menstruation.

A dysfunction at any point can cause menstrual problems, including amenorrhea. For example, a girl who has no vagina will not menstruate. Although this may seem unlikely, it is a relatively common cause of primary amenorrhea. Approximately 12 percent of patients with primary amenorrhea will have this condition, in which the vagina is either absent or in a state ofarrested growth.

The most common causes of primary amenorrhea are in-born defects of the uterus, ovaries or vagina that are caused by chromosomal abnormalities. One of these is testicular feminization syndrome (also called pseudohermaphroditims), in which a child who appears to be a girl actually is a boy-that is, the chromosomes are XY instead of XX. Such children have normal female genitals but only a partially developed vagina and no uterus. They also have underdeveloped tests that may go unnoticed until a close examination is made.

Another condition traced to chromosomal abnormality is Turner's syndrome, caused by complete or partial absence in the female of one of the two X chromosomes. Girls with this condition have underdeveloped ovaries incapable of developing ova--or of secreting the hormones necessary to maintain the menstrual cycle.

Although primary amenorrhea is most often caused by genetic abnormalities and the resulting defects in the sexual organs, secondary amenorrhea usually has its roots in what is known as the hypothalamic-pituitary-ovarian axis (HPO axis). The HPO axis is the brain-to-ovaries communication, carried out by the various hormones (LH, FSH, estrogen and progesterone). If something disrupts communications, a result can be failure to menstruate.

Many women who complain to their doctors of secondary amenorrhea have polycystic ovary syndrome, in which one or both ovaries enlarge and thicken. It is a consequence of persistent anovulation (failure to ovulate), which eventually manifests itself as amenorrhea. Since there are many causes of anovulation--most rooted in the hypothalamus and pituitary--there are also many types of polycystic ovaries. Other symptoms of polycystic ovary syndrome are obesity, infertility and hirsutism (abnormal growth of hair), particularly on the face.

Anovulation can be caused by tumors of the pituitary gland, called adenomas, which are responsible for about a third of all secondary amenorrhea cases. The most common is the prolactin-secreting tumor.

Prolactin, secreted by the pituitary gland, stimulates the secretion of breast milk. This hormone is released in response to suckling (as nerve signals from the breasts travel to the hypothalamus, which in turn sends its signal to the pituitary gland). Excessive prolactin can cause both amenorrhea and anovulation. That's why some 50 percent of nursing mothers do not menstruate. A prolactin-secreting pituitary tumor can cause the inappropriate release of excessive prolactin, a condition called hyper-prolactinemia.

Ordinary menstraual irregulatries are frequently caused by physical and mental stresses that throw the hypothalamus slightly out of adjustment, but sometimes these same stresses can also lead to amenorrhea. Located in the center of the brain, the hypothalamus might be considered as an information processing center, both receiving and sending signals vital to the body. As part of the nervous system, it is sensitive to stimuli collected through the five senses. It also stimulates release from the pituitary of hormones that control many of the body's functions (including eating, sleeping, and sexual activities).

Travel, change in climate or sleep habits, and mental distress all can affect menstrual regularity. If the stresses are great enough, the clinical condition of amenorrhea can be the result. Dr. Hilde Bruch, professor of psychiatry at Baylor College of Medicine in Houston, says in her book Eating Disorders, that "amenorrhea is commonly observed in women under severe stress and strain . . . . Under wartime conditions, in concentration and internment camps, incidence of amenorrhea was high . . . ." Likewise, women with mental problems such as those in mental institutions can lose their menstrual cycles.

Hypothalamic amenorrhea also can be caused by acute loss of weight, as may occur in crash dieting. Dr. Rose E. Frisch, of the Harvard Center for Population Studies, Cambridge, Mass., holds that regular menstrual periods depend on the maintenance of minimum weight to height. She says amenorrhea results from a loss of about 10 to 15 percent of body weight (about one-third of body fat).

There is some evidence that loss of body fat leads to dysfunction of the hypothalamus and subsequent reduction in FSH and LH, which in turn causes failure to ovulate. Frisch claims this is an adaptive mechanism that prevents a woman from having a baby if she hasn't enough calories to nurture the fetus; not all authorities agree with this theory.

Amenorrhea is a common symptom of anorexia nervosa, a psychiatric disease characterized by aversion to eating and extreme weight loss. Patients with this condition--usually adolescents and women in their early twenties--suffer from unrealistic concerns about gainting weight and about how they look. Even when emaciated to a point requiring hospitalization, victims frequently insist that they are "too fat".

Most medical experts say that the weight loss in anorexics causes the amenorrhea. Some claim, however, that the amenorrhea in such patients preceedes the weight loss. They believe an abnormality in the hypothalamus causes both the disease and the loss of menstrual periods.

Another cause of hypothalamic amenorrhea consistent with weight loss in strenuous physical activity. Dr. Phil Price, gynecologist with FDA's Center for Drugs and Biologics, says amenorrhea is occuring more frequently now in women who run long distances, as in marathons. He notes, however, that it doesn't seem to become a problem until a woman begins running about 20 miles a week or more, and even then it won't necessarily occur.