Ovulation is the process of discharging a mature ovum (egg) from an ovary after a Graafian follicle - representing the final stage of follicular development before ovulation - has been formed.

In humans, ovulation happens approximately 14 days before the start of the next menstrual cycle. However, normal ovulation can occur as early as around day 8, or as late as day 20 or beyond.

The timing of ovulation depends on the length of the woman's menstrual cycle, hence a woman with a particularly short cycle will ovulate earlier while a woman with a long cycle will ovulate much later. Some women can feel a slight pain in their lower abdomen, known as Mittelschmerz or "intermenstrual pain".

In every menstrual cycle some ovarian follicles begin maturation, but mostly only one will end as a Graafian follicle, growing rapidly under the influence of FSH and LH at the end of its development to a diameter of approximately 15 mm, forming a "blister" on the surface of the ovary with a small spot without blood vessels, the "stigma".

Enzymatic processes and the growing pressure of its fluid-filled antrum lead to its rupture.

By this time, the oocyte has undergone its first meiotic division, leading to two cells of very different size: the secondary oocyte - containing all of the cytoplasmic material of both daughter-cells - and the first polar body.

The second meiotic division follows at once without DNA replication but will be arrested in the metaphase and will so remain until fertilization. The spindle apparatus of the second meiotic division appears at the time of ovulation. If no fertilization occurs, the oocyte will degenerate approximately 24 hours after ovulation.

At ovulation, the mucous membrane of the uterus - termed the functionalis - has reached its maximum size, and so have the endometrial glands, although they are still non-secretory.

Knowing about the time of ovulation is critical for all forms of natural family planning, though the rhythm method is not typically considered a reliable method of birth control.

Recent research suggests that in some women, ovulation may occur more than once during each menstrual cycle, making it difficult for a woman to be aware of exactly when she is fertile.

This discovery is considered by some to be a likely explanation for pregnancies caused by sexual intercourse when a woman is sure she should not be ovulating. Another explanation is, of course, that the woman has mistakenly assumed that she ovulates on day 14, when in fact, she ovulates earlier or later in her cycle.