What causes menstrual cycle irregularities?

Menstrual cycle irregularities can have many different causes, including:

• Pregnancy or breast-feeding. A missed period can be an early sign of pregnancy. Breast-feeding typically delays the return of menstruation after pregnancy.
• Eating disorders, extreme weight loss or excessive exercising. Eating disorders — such as anorexia nervosa — extreme weight loss and increased physical activity can disrupt menstruation.
• Polycystic ovary syndrome (PCOS). Women with this common endocrine system disorder may have irregular periods as well as enlarged ovaries that contain small collections of fluid — called follicles — located in each ovary as seen during an ultrasound exam.
• Premature ovarian failure. Premature ovarian failure refers to the loss of normal ovarian function before age 40. Women who have premature ovarian failure — also known as primary ovarian insufficiency — might have irregular or occasional periods for years.
• Pelvic inflammatory disease (PID). This infection of the reproductive organs can cause irregular menstrual bleeding.
• Uterine fibroids. Uterine fibroids are noncancerous growths of the uterus. They can cause heavy menstrual periods and prolonged menstrual periods.