You do not have to be sexually active to experience amenorrhea. Although pregnancy is one of the most common causes of amenorrhea, there are many other conditions that can cause a lack of menstruation. Primary amenorrhea is diagnosed when menstruation has not started by age 16 and is usually caused by a genetic disorder or birth defect that stifles the effects of puberty. Other causes include: malnutrition, excessive exercise or excessive stress, illness, certain drugs, hormonal imbalances, uterine scarring, or even premature menopause. See your doctor about treatment options for any of these causes.