Although few women menstruate exactly every 28 days, extremely short (under 25 days) or long (over 34 days) cycles can be a cause for concern, especially if you plan to have children.

Short cycles often signal low levels of estrogen and progesterone in the system, possibly resulting from an undersupply of certain precursors. Lacking these hormones, the endometrium cannot develop properly, and infertility may result. Short cycles also develop as some women approach menopause. They can also result from over- or underactivity of the thyroid gland.

Irregular periods can be a sign of appproaching menopause. But they can also arise from an increase in the number of cells in a section of the endometrium. Endometrial hyperplasias are caused when too much estrogen is produced by a women who does not ovulate. To diagnose the problem, your doctor will probably perform a D&C, scraping cells from the endometrium and doing a biopsy. Mild hyperplasias are usually treated with monthly doses of progesterone. More serious hyperplasias require long-term progesterone therapy or even removal of the uterus.

Long cycles are not necessarily a problem. Many women with long cycles produce eggs and are fertile. Their ovaries are normal, and the eggs just take a long time to mature. By far the most common cause of an unexpectedly long cycle is simply pregnancy! However, some women with regular periods two to five months apart may have ovarian cysts. Also, when a very long cycle is accompanied by a sudden increase in body hair, a decrease in breast size, and enlargement of the clitoris; and menstruation eventually stops altogether, the problem could be a growth or tumor of the adrenal gland. To make a diagnosis, the doctor will take urine, glucose tolerance and other tests. A CT scan or Magnetic Resonance Imaging (MRI) might also be ordered.

Long cycles can also develop from over or underproduction of thyroid hormone.