Menstruation

The reproductive cycle that begins when an ovary releases an egg. At the same time, estrogen is released, stimulating the lining of the uterus to thicken with blood. If an egg is not fertilized, progesterone levels drop and blood vessels constrict, the uterine lining sheds, and the menstrual flow begins. This cycle repeats itself about every 28 days until interrupted by pregnancy or ended by menopause.

Normally, menstruation begins at some point between the ages of 11 and 13, although a delay through age 17 is still considered to be normal. A girl who is older than 17 and has never had a menstrual period should see a doctor. Lack of weight gain, stress, or hormone irregularities all can contribute to a delayed period.