Ovulation is a complicated process, involving several biological and chemical steps. Consequently, ovulation may be interrupted at several stages, and many different conditions and events can arrest ovulation.
The typical end of ovulation in humans is called menopause. Menopause occurs 12 months after a woman's last period--in the Western world, around age 45 to 55. This is the most common explanation for ovulation to stop completely. It is a natural and inevitable part of living a full life as a woman, not a disease.
If ovulation is arrested before menopause it is called anovulation. Women who consistently do not release a mature egg once per menstrual cycle are called anovulatory. This is usually the result of a treatable condition. A significant number of sexually mature women--between 6 and 15 percent--are anovulatory.
Causes of Anovulation
Anovulation can be caused by numerous conditions. Sudden weight loss, polycystic ovarian syndrome (PCOS) and problems with the thyroid, pituitary gland, or hypothalamus are the most typical causes of anovulation. Sudden weight loss--as when a woman is starving or anorexic--can in some cases cause the body to shut down ovulation. It is thought that this may be an evolved mechanism to protect both the health of the woman and the future of the species as a whole (better to save eggs for times of plenty; better to not run the risk of a starving woman becoming pregnant).
PCOS describes the appearance of numerous cystic bodies on the ovaries. These masses prevent the formation of the ovarian follicle and can lead to irregular periods and anovulation. Usually a hereditary condition, PCOS is responsible for up to 90 percent of anovulation cases.
The majority of non-PCOS anovulation is attributable to hormonal imbalance. A balance of hormones in the body is necessary for egg release and follicle formation, so significant hormonal disease can cause infertility. Typically this is the result of damage to the thyroid or pituitary glands. The thyroid produces hormones necessary for proper ovulation, while the pituitary gland controls the ovaries themselves. Another possibility is damage to or disease in the hypothalamus, which, in turn, controls the pituitary.
Less common causes of anovulation include obesity, hirsutism, drug use, and physical damage to the ovaries themselves (ovarian dystrophy).