I am a 20-year-old woman and I have not yet started my period. Could there be something wrong? I know women start at different ages, but I am starting to worry. I am slightly overweight. I try to watch my diet, but I don't exercise like I should. Should I go get my period started, or wait for nature to take its course?

--T.L.

It is abnormal if menstruation has not begun by age 16, so at age 20, you definitely need evaluation by a gynecologist. There are several possible reasons for primary amenorrhea, the medical term for never starting your period. Most fall into one of four categories: disorders of the uterus or vagina, disorders of the ovary, disorders of the pituitary gland, and disorders of the hypothalamus or central nervous system.

-- Disorders of the uterus or vagina (called "outflow tract disorders") include blockages in or even complete absence of those organs. There are several possible sources of blockage in the vagina. Sometimes the hymen, the membrane that covers the vaginal opening in a virgin, does not have the normal small opening that lets blood flow out; as a consequence, menstrual blood is trapped in the vagina and uterus. Other times, there is a septum (wall of tissue) that blocks the vagina higher up. Both of these conditions are treated by vaginal surgery.

Some women are born without a vagina at all; often, the uterus has not formed either. These women have normal ovaries and normal hormonal function, but they can never become pregnant. They can, however, use a surrogate mother to carry a child conceived from their eggs and their partner's sperm. Women with a congenitally absent vagina can have a satisfying sexual life after a vagina is created through surgery or the use of special dilators.

A final outflow tract disorder is a hormonal problem called androgen insensitivity or testicular feminization. This is the third most common cause of primary amenorrhea. A woman with androgen insensitivity actually has the genes to be a male, but due to an defect in the processing of testosterone, male sexual organs never develop, and she appears female, with a vagina, breasts and little body hair. However, her vagina ends blindly, and she has no uterus or ovaries at all.

-- Abnormalities of the ovary leading to lack of periods can include being born with "streak" ovaries -- little tiny nubbins of nonfunctional ovarian tissue. This may occur in the genetic condition called Turner's syndrome, in which a woman only has one X chromosome instead of the normal two. Women with Turner's are infertile, but hormone replacement can bring on normal periods.

A few young women will have premature ovarian failure -- that is, very early menopause. Sometimes this is due to autoimmune causes (the body attacking itself, to put it simply). Sometimes in such cases, ovarian function returns on its own. Ovarian failure and resultant primary amenorrhea may also occur as a consequence of extremely high radiation doses or high doses of certain types of chemotherapy drugs used to treat cancer.

-- Disorders of the pituitary gland, a small organ located in the brain, between the eyes, can affect a wide range of body functions. Small tumors, called microadenomas, secrete high levels of the hormone prolactin; high levels of prolactin can interfere with ovulation and prevent periods from occurring. Most of the time, these tumors are very responsive to medication, and treatment restores regular menses and fertility. In other cases, the pituitary gland may die off as the result of hemorrhagic shock. However, this is usually associated with massive bleeding after delivering a baby, and is not a consideration in your case.

-- Disorders of the hypothalamus, another part of the brain, may result from such factors as psychological stress, anorexia or excessive exercise. A malfunction of the hypothalamus, the "master gland," can cause disruptions in all of the reproductive hormones. Periods stop -- or never start, if such events occur in puberty. Normalization of weight and body fat, or easing of the stress, often brings with it a return of normal menstrual function.

A gynecologist can evaluate the cause of your lack of periods. A physical exam and blood work will usually make the diagnosis, and then appropriate treatment can be given. This should not be ignored.