Excessive weight loss or gain. Although low body weight is a common cause of missed or irregular periods, obesity also can cause menstrual problems. Eating disorders, such as anorexia or bulimia. For more information, see the topic Anorexia Nervosa or Bulimia Nervosa. Increased exercise. Missed periods are common in endurance athletes. Emotional stress. Illness. Travel. Medicines such as hormonal birth control methods, which may cause lighter, less frequent, more frequent, or skipped periods or no periods at all. Hormone problems. This may cause a change in the levels of the hormones that the body needs to support menstruation. Illegal drug use. Problems with the pelvic organs, such as imperforate hymen, polycystic ovary syndrome, or Asherman s syndrome. Breast-feeding. Many women do not resume regular periods until they have completed breast-feeding.