What's the menstrual cycle?

The menstrual cycle is the monthly series of changes a woman's body goes through in preparation for the possibility of pregnancy. Each month, one of the ovaries releases an egg — a process called ovulation. At the same time, hormonal changes prepare the uterus for pregnancy. If ovulation takes place and the egg isn't fertilized, the lining of the uterus sheds through the vagina. This is a menstrual period.

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.