In medicine, anovulation is absence of ovulation when it would be normally expected (in a post-menarchal, premenopausal woman). Anovulation can result from a variety of factors:

• Stress, new environment
• Chronic mental illness, such as depression
• Chronic physical illness, such as inflammatory bowel disease, poorly controlled diabetes, tuberculosis, or anemia
• Undernutrition, specific nutrient deficiencies, inadequate body fat
• Prolonged or continuous physical exertion
• Various pharmaceutical (especially phenothiazines) and recreational drugs
• Hormone imbalances, such as prolactin or testosterone excess (e.g., polycystic ovary syndrome), hyper- or hypothyroidism, adrenal insufficiency or Cushing's syndrome.
• Pituitary or ovarian failure.

Some anovulatory women may have two or more contributing conditions. Anovulation can generally be reversed by ameliorating the causal factors (except in cases of permanent pituitary or ovarian failure).

For most women, alteration of menstrual periods is the principal indication of chronic anovulation. Ovulatory menstrual periods tend to be regular and predictable in terms of cycle length, duration and heaviness of bleeding, and other symptoms. Ovulatory periods are often accompanied by midcycle symptoms such as mittelschmerz or premenstrual symptoms. In contrast, anovulation usually manifests itself as irregularity of menstrual periods, that is, unpredictable variability of intervals, duration, or bleeding. Anovulation can also cause cessation of periods (secondary amenorrhea) or excessive bleeding (dysfunctional uterine bleeding). Mittelschmerz and premenstrual symptoms tend to be absent or reduced when a woman is anovulatory.

Chronic anovulation is a common cause of infertility.

In addition to the alteration of menstrual periods and infertility, chronic anovulation can cause or exacerbate other long term problems, such as hyperandrogenism or osteopenia. It plays a central role in the multiple imbalances and dysfunctions of polycystic ovary syndrome.