Each time a woman ovulates, she releases an egg. By definition ovulation is the release of a mature egg from an ovary during the menstrual cycle.
Ovulation usually occurs 12 to 16 days after the first day of a woman's last period. An egg, or ovum, lives only 12 to 24 hours after leaving the ovary. During this window of time, sperm is able to fertilize the egg if they meet.
Ovulation does not always occur during its normal time in the menstrual cycle. Increased stress, poor health and other disruptions sometimes delay or inhibit ovulation. An "anovulatory" menstrual cycle is one in which ovulation is skipped.