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 symptom of pregnancy. Breast-feeding typically delays the return of your period after pregnancy.
• Eating disorders, extreme weight loss or too much exercising. Eating disorders — such as anorexia nervosa — extreme weight loss and higher physical activity can interrupt your period.
• Polycystic ovary syndrome (PCOS). People with this common disorder may have irregular periods. They also can have enlarged ovaries that contain small collections of fluid — called follicles — located in each ovary. These follicles can be seen during an ultrasound exam. People who have PCOS often have more follicles in the ovaries than other people.
• Premature ovarian failure. Premature ovarian failure refers to the loss of typical ovarian function before age 40. People who have this condition, 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 growths in the uterus that are not cancer. They can cause heavy and prolonged menstrual periods.