The most common reason for bleeding twice in one calendar month occurs when a normal period begins on the first or second day of the month, and then another arrives at the very end; because the normal cycle length (from the first day of one to the first day of the next) ranges from 23 to 35 days, this can allow two perfectly normal periods to arrive in the same calendar month. Most of the time, however, when women talk about having two periods in a month, they are not referring to this circumstance. Often they mean they are starting a period every two weeks or so -- certainly less than the 23-day interval that is considered within the normal range.
Anything that can disrupt the delicate hormonal dance that produces ovulation can lead to either skipped periods or periods that come too often. Hormonal imbalance as a cause of frequent periods is often seen as a woman nears menopause. Thyroid dysfunction may also be a factor.
Another reasons for twice-a-month periods is abnormal thickening of the uterine lining. This thickening can be due to hormonal imbalance or to taking estrogen without a progesterone medication as well; estrogen causes the uterine lining to proliferate and become thick and lush, while progesterone thins out the lining; a balance between these two hormones is necessary to ensure regular, moderate periods.
Polyps (projections of tissue that protrude into the uterine cavity) or fibroids that impinge on the uterine cavity may also cause irregular periods. In this case, the uterine lining tissue that overlies these projections is less stable, increasing the chance that it will be shed at an inappropriate time. Also, fibroids and polyps increase the surface area of the uterine lining, often leading to prolonged and heavier bleeding.
If your periods are coming less than 23 days apart (counted from the beginning of one to the beginning of the next), then you should see your gynecologist. A biopsy may be necessary to rule out an abnormal thickening of the uterine lining. Blood tests may be done to assess hormone levels. Most of the time the reason for frequent bleeding is benign, but occasionally the problem can be more serious; for this reason, and because excess bleeding can lead to anemia, too-frequent periods must be investigated.
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