Amenorrhea (ah-men-o-REE-ah) is the absence of menstrual periods. There are two types of amenorrhea, primary and secondary. In primary amenorrhea, menstruation has never occurred at all. In most cases, this is simply due to the late onset of puberty. If, however, a girl has not menstruated by the age of 16, it's recommended she visit a physician for a gynecological examination, to rule out possible problems. Other causes of primary amenorrhea include anemia (uh-NEE-mee-uh), and disorders of the uterus, ovaries, or pituitary (pih-TOO-ih-tare-ee) gland. Sometimes there's a false primary amenorrhea during which the patient is menstruating, but the blood is prevented from reaching the outside due to a blockage of some type. Secondary amenorrhea occurs in women who go four months or more between periods, after their regular menstrual cycle has been established. The most common cause of secondary amenorrhea is pregnancy. However, menstrual patterns can also be altered by anemia, emotional stress, illness, certain medications, oral contraceptives, ovarian cysts, and excessive exercise. Female athletes and ballet dancers often have infrequent periods, or may stop menstruating completely. In part, the cause is thought to be low levels of body fat. The severe weight loss of anorexia is known to stop menstruation. If you develop secondary amenorrhea, and you're not pregnant, it's possible that the timing of your menstrual cycle is merely off. However, if you experience the absence of regular periods, it's recommended that you consult your health care provider. While lack of bleeding alone is not necessarily harmful, it can signal a low level of estrogen, which can eventually cause bone loss.