Menstruation involves numerous structures and organs, including the hypothalamus, pituitary gland, ovaries, uterus, cervix and vagina. Menstrual disorders may occur when certain diseases or conditions interrupt the normal function of these structures.

Common causes of menstrual disorders include:

• Polycystic ovarian syndrome (PCOS). A reproductive disorder in which excessive amounts of androgens (male hormones) are produced by the ovaries. PCOS may cause a number or menstrual disorders including amenorrhea, oligomenorrhea (unusually infrequent periods) and polymenorrhea. For many women with PCOS, the menstrual disorders are caused by anovulation, or failure to ovulate, leading to fertility issues as well.
  
  
• Pregnancy and pregnancy complications. Pregnancy is the most common cause of primary amenorrhea. In addition, menorrhagia (very heavy periods) may result from miscarriage (spontaneuous loss of pregnancy before 20 weeks of gestation) or ectopic pregnancy (a pregnancy implanted outside the uterus).
  
  
• Certain forms of contraception. Starting or stopping the use of birth control pills can result in amenorrhea. Some types of oral contraceptives cause a very light or nonexistent menstrual flow. Others are designed to restrict menstruation to a specific number of cycles per year. Intrauterine devices (IUDs) can cause menorrhagia in some women, or hypomenorrhea (unusually light menstrual periods) and even amenorrhea in women using a progesterone-releasing IUD.
  
  
• Uterine disorders. Uterine fibroids (noncancerous tumors or growths within the lining of the uterus) can cause secondary dysmenorrhea, menorrhagia or metrorrhagia. Uterine polyps can cause metrorrhagia, menorrhagia or postmenopausal bleeding. Hyperplasia (thickening of the uterine lining) is a common cause of postmenopausal bleeding. Adenomyosis (condition in which glands from the endometrium become implanted in the uterine muscle) may cause unusually heavy periods.
  
  
• Endometriosis. A disorder in which the tissue that lines the uterus becomes implanted outside the uterus. The condition can lead to secondary dysmenorrhea or menorrhagia.
  
  
• Pelvic inflammatory disease (PID). An infection of the reproductive organs. PID may lead to secondary dysmenorrhea or menorrhagia.
  
  
• Thyroid disorders. Having an overactive thyroid gland (hyperthyroidism) can cause amenorrhea, and having a slow thryroid (hypothyroidism) can lead to menorrhagia.
  
  
• Gynecologic cancers and cancer treatment. Cancer of the uterus, ovaries or cervix can cause metrorrhagia (infrequent periods) or menorrhagia (heavy periods). Postmenopausal bleeding is often the result of cervical, vaginal or endometrial cancer. In addition, chemotherapy treatments can cause a woman to develop menorrhagia. Certain types of chemotherapy can also put a woman into a medical menopause and cause amenorrhea.

Additional causes include:

• Birth defects. Primary amenorrhea can result from abnormalities of the vagina or lack of reproductive organs. These congenital disorders may go undiagnosed until puberty.
  
  
• Breastfeeding. Among the most frequent causes of secondary amenorrhea is breastfeeding.
  
  
• Low body weight and excessive exercise. A low body weight can interfere with pubertal development and prevent menstruation. Women who exercise intensely (e.g., ballet dancers, gymnasts, long-distance runners) may lower the body weight enough to prevent menstruation. Women with eating disorders (e.g., anorexia nervosa) can also affect the body’s ability to function normally, preventing menstruation.
  
  
• Stress. Strong emotions such as stress can impact the body and prevent or delay menstruation.
  
  
• Hormone-secreting tumors. Certain tumors secrete hormones that interfere with the menstrual cycle, causing either absent or infrequent periods.
  
  
• Chronic illness (e.g., diabetes, lupus, liver disease, kidney disease). Certain diseases can affect the menstrual cycle causing heavy, infrequent or absent menstrual periods.
  
  
• Sexually transmitted diseases (STDs). Infection with certain STDs (e.g., gonorrhea) can cause irregular but frequent bleeding.
  
  
• Medications. The use of certain medications (e.g., anticoagulants, anti-inflammatory medications) may cause hormonal imbalances, resulting in heavy bleeding.
  
  
• Postmenopausal bleeding. Vaginal bleeding in postmenopausal women may result from atrophic vaginitis (thinning of the vaginal lining).
  
  
• Premenstrual syndrome (PMS). The exact cause of PMS is not known. However, possible causes include hormonal changes, chemical changes and diet.
  
  
• Bleeding disorders. Certain disorders of the coagulation (blood clotting) system, such as Von Willebrand’s  disease, can present with excessive menstrual bleeding.

In addition, hypomenorrhea (extremely light menstrual periods) can be a normal finding in some women. It can result from certain medications, most notably oral contraceptive pills are known to significantly reduce menstrual flow in certain women. Hypomenorrhea can also result after a woman has had certain surgical procedures aimed at reducing heavy menstrual bleeding, such as an endometrial ablation.

Menometrorrhagia and polymenorrhea are usually the result of either a structural or hormonal abnormality. Structural abnormalities that can cause frequent and/or heavy menstrual bleeding include uterine fibroids, polyps or tumors. Irregular growth of the endometrium caused by hormonal irregularities is the most common cause of heavy menstrual bleeding. Complications of pregnancy, including miscarriage, can also cause heavy bleeding.