When periods stop in a sexually active, regularly menstruating woman, the first thing that comes to mind is a possible pregnancy. If she is past 40, menopause may be the cause. But when the doctor has ruled out both pregnancy and menopause, it's time to look for other reasons.

This condition bears the medical name secondary amenorrhea. It is defined as the lack of a period either for six months or for at least three times as long as the length of a menstruating woman's normal cycle. Causes range from tumors and cysts to weight gain or loss, and emotional factors.

Chronic failure to ovulate is one of most common causes. A lack of ovulation is normal during the first couple of years after menstruation begins and again before menopause. But at other times it may be due to low levels of a key reproductive hormone called GnRH (gonadotrophic releasing hormone). Levels of this hormone often drop when a woman is under stress, has been on a “crash” diet, has had a head injury or serious infection such as encephalitis or meningitis, or has stopped using birth control pills.

If the doctor suspects that lack of ovulation is the culprit, he or she will ask you to record your temperature upon waking . The doctor will study samples of your cervical mucus and vaginal secretions and examine a piece of tissue from the endometrium. He or she may also need to determine whether your progesterone level rises over the course of a month.

If failure to ovulate is indeed the problem and you don't want to become pregnant, the doctor will prescribe estrogen and progesterone or an oral contraceptive. This promotes shedding of the endometrium and discourages development of growths in the uterus that can occur when estrogen levels remain high for a long period of time. If you do want to have a baby, other medications are usually prescribed.

Several problems with the ovaries can also cause periods to stop. To check the ovaries, your doctor may ask you to begin taking progesterone. If you fail to menstruate after seven days it's an indication of inadequate estrogen levels, a possible pregnancy, or a disruption in the ovarian cycle. The doctor may also study vaginal secretions, which can show whether the ovaries are wasting away, hardened, or are able to function normally. Other tests can tell whether development of the ovaries is normal and whether they are producing estrogen properly.

Often, ovarian cysts are at fault. Together with a thickened endometrium, they are the hallmark of a condition called the Stein-Leventhal syndrome. Women with this problem fail to menstruate, may fail to ovulate (or ovulate only occasionally), have a great deal of facial and/or body hair, and may have episodes of heavy bleeding between bouts of amenorrhea. Many of these women have increased levels of testosterone, a male sex hormone normally present in small quantities in the female as well.

To diagnose the problem, the doctor will determine the levels of androgen and estrogen through laboratory tests. He or she will also examine the pelvic area to see whether the ovaries are enlarged due to the presence of cysts.

If the doctor finds a number of cysts, and you do not want to become pregnant, he or she will prescribe Provera, or birth control pills to cause the endometrium to shed. A combination of estrogen and progesterone will suppress ovarian function, and thus decrease the risk of cancer of the endometrium. If you want to conceive, the doctor may give you Clomid or Pergonal to induce ovulation.

• Problems in the uterus and fallopian tubes may be to blame for amenorrhea. In some cases the lining of the uterus continues to grow unchecked for many weeks or years. Women with this condition may have one or two months without a period preceded and followed by excessive bleeding. A D&C (scraping of the uterus) and biopsy of the lining may be necessary for diagnosis. To treat the problem, the doctor will prescribe Provera or estrogen-progesterone therapy.

• Malfunctioning adrenal glands that secrete excessively high or low levels of adrenal hormone can also lead to amenorrhea. Tumors, steriod therapy, and even weight loss can all affect adrenal performance. Prednisone, dexamethasone, and hydrocortisone can often clear up the problem. Girls who are born with malfunctioning adrenals must have lifelong treatment. In most other cases, the condition clears up and treatment can be discontinued after several months.

• Other glandular disorders can also be at fault. Cysts, tumors, serious infection, and eating disorders can disrupt the pituitary gland and lead to amenorrhea.

• Overactivity or underactivity of the thyroid gland can cause the problem, too. To correct specific glandular imbalances, there are a number of medications your doctor can prescribe.

• Anorexia nervosa, the loss of more than 25 percent of one's ideal body weight, is another potential cause of amenorrhea—as well as other serious physical and emotional problems. Almost all anorexic women stop menstruating, and many have glandular disorders leading to low levels of estrogen. If anorexia is the culprit, you'll need treatment for the underlying problem as well as the lack of periods.

• Breastfeeding women may fail to menstruate for 10 or more months. Their high levels of prolactin, a hormone necessary to produce breast milk, may suppress the hormones that trigger the menstrual cycle. Since ovulation is still a possibility, all breastfeeding women should use “barrier” birth control such as a diaphragm or vaginal sponge if they want to prevent conception.

• There is no substantial proof that either prolonged use of an oral contraceptive (the “Pill,”) or use at an early age causes amenorrhea. Close to 95 percent of nonmenstruating users of oral contraceptives resume normal cycles spontaneously after discontinuing the medication. The one percent who fail to menstruate for more than six months after stopping the pill generally have a glandular or ovarian disorders.

Some women stop menstruating permanently before they reach the age of 35 and begin to experience the typical symptoms of menopause. Their ovaries secrete insufficient estrogen to maintain the menstrual cycle and become small and wasted away. There is no effective treatment for this condition. Progesterone will not cause a return of menstruation, and in all but a few instances, drug therapy will not restart ovulation.