The absence of menstruation. Primary amenorrhea is the absence of the onset of the first menses in an adolescent, with or without pubertal changes. Secondary amenorrhea is the absence of menses for at least 6 months or three cycles in a woman with previously normal cycles. The most common reasons are pregnancy and stress, but significant illness can lead to secondary amenorrhea as well. Temporary amenorrhea may occur after stopping oral contraceptives. About 90% of all women who have ovulated regularly before taking the pill will have again begun to do so by three months after stopping oral contraceptives.