Absence of a woman's monthly menstrual period is called amenorrhea. Secondary amenorrhea is when a woman who has been having normal menstrual cycles stops getting her periods for 6 months or longer.
Secondary amenorrhea can occur due to natural changes in the body. For example, the most common cause of secondary amenorrhea is pregnancy. Breastfeeding and menopause are also common, but natural, causes.
Women who take birth control pills or who receive hormone shots such as Depo-Provera may not have any monthly bleeding. When they stop taking these hormones, their periods may not return for more than 6 months.
You are more likely to have absent periods if you:
Exercise too much and for long periods of time
Have very low body fat (less than 15 to 17%)
Have severe anxiety or emotional distress
Lose a lot of weight suddenly (for example, from strict or extreme diets or after gastric bypass surgery)
Other causes include:
Brain (pituitary) tumors
Drugs for cancer treatment
Drugs to treat schizophrenia or psychosis
Overactive thyroid gland
Polycystic ovarian syndrome
Reduced function of the ovaries