Amenorrhea is the absence of menstruation or having periods. It is classified once a woman has missed at least three consecutive periods or who has not started menstruating within the normal age range.
Primary amenorrhea is when a girl hasn't started menstruating before the ages of 15-16. There might not be any concern but a doctor should be consulted just in case there is an underlying reason for the delay.
Secondary amenorrhea is when menstruation isn't occurring in a woman in the absence of pregnancy, lactation, menopause, or birth control medications that stop periods. There can be several causes of amenorrhea including pregnancy, reproductive organ issues, hormonal issues, and birth control medication side effects.
Menstruation can also cease in women who have a shortage of body fat (this effects some female athletes as well as women suffering from anorexia). In order to have regular menstruation, the hypothalamus, pituitary gland, ovaries, and uterus must all be functioning properly. Bulimia nervosa and anorexia nervosa can also cause amenorrhea in individuals.