Ovulation occurs when an egg moves from the ovaries into the fallopian tubes and is ready for fertilization. It occurs midway in a woman's menstrual cycle, but the timing varies for each woman.  

Menstrual cycle

At 20 weeks old, a female fetus has 2 million immature eggs called oocytes inside her ovaries, but she loses 75 percent of these eggs by the time she is born, according to The Center for Menstrual Disorders & Reproductive Choice. This leaves most females with half million immature eggs. The eggs are fully matured by the time the female enters puberty.

When an egg moves into the fallopian tubes, a sperm cell can fertilize the egg, which could then move into the uterus, or womb, and develop into a fetus.

During ovulation, the walls of the uterus also thicken to prepare for a fertilized egg, but if the egg is not fertilized, the uterus sheds that lining, causing the monthly bleeding of a menstrual period. Having a period does not always equal ovulation, though.

"The most misunderstood thing about ovulation is the idea that if you are menstruating, it means that you are ovulating; and that is indeed not the case at all," said Dr. Mary Jane Minkin, a clinical professor of obstetrics, gynecology and reproductive sciences at the Yale School of Medicine.

Many women have an ovulatory cycle — the buildup of the lining of the uterus — because they are making estrogen. But when it gets to a certain height, the lining just sloughs off, and a woman can bleed quite heavily, said Minkin. When a woman ovulates she makes the hormone progesterone, which results in a more controlled bleed.

When a woman is getting a period, but not getting pregnant, it may be because she is not ovulating. An ovulation predictor kit can be helpful to see if a woman is indeed ovulating. "And if it shows that ovulation isn't occurring, it's a great time to check in with your gynecologist: because getting women to ovulate is often quite straightforward," said Minkin.

Ovulation problems

There are many reasons why a woman may have ovulation problems. Some women, for example, have blocked fallopian tubes due to pelvic inflammatory disease, endometriosis or surgery for an ectopic pregnancy, according to U.S. Department of Health and Human Services.Polycystic ovary syndrome (PCOS), thyroid problems and other conditions can make the ovaries less likely to produce an egg.

Problems with ovulation are just one possible cause for infertility. About 12 percent of women (more than 6.1 million) in the United States between ages 15 to 44 have difficulty getting pregnant or staying pregnant, according to the Centers for Disease Control and Prevention (CDC). Physical problems with the uterus and uterine fibroids can be some other causes for infertility.

Timing

Many people mistakenly believe ovulation always happens exactly 14 days after a woman's last period. But the timing varies for each woman, usually falling between days 11 and 21 of the menstrual cycle. "Most women have no idea when they ovulate," said Dr. Christina Ramirez, an obstetrician and gynecologist in Garden City, New York.

Mammals other than humans and apes go through an "estrous cycle" instead of a menstrual cycle, according to the Encyclopedia Britannica. In this cycle, females are only sexually active during their estrous phase — sometimes called "being in heat." 

Both human and ape females can be sexually active at any time in their cycles. However, mammals with estrous cycles don't have menstrual periods, because the uterus reabsorbs its lining instead of shedding it.

Missed periods

Amenorrhea is the absence of one or more missed menstrual periods, according to the Mayo Clinic. While pregnancy might be the first thing that comes to mind when a period is late, it isn't the only reason menstruation might be delayed. There are many factors that may contribute to the disruption of a woman's menstrual cycle.

The menstrual cycle is regulated by the endocrine system, which releases hormones — including the sex hormones estrogen and progesterone — into the body. These hormones play a key role in all stages of the menstrual cycle, allowing the ovum (egg) to mature and eventually be released into the uterus.

But many different disorders can result in hormonal imbalances and delayed menstruation, including hypothyroidism (underactive thyroid gland), hyperthyroidism (overactive thyroid gland), pituitary adenoma (noncancerous tumor of the pituitary gland) and adrenal insufficiency (a condition in which the adrenal glands do not produce adequate amounts of certain hormones).

Emotional or physical stress can also delay ovulation and, therefore, menstruation. Severe stress causes a decrease in the amount of gonadotropin-releasing hormone (GnRH) produced by the body, according to the University of California, Berkeley. The right amount of GnRH — which controls the release of hormones such as follicle-stimulating hormone (FSH) and estrogen — is needed for ovulation. Stress in the weeks leading up to ovulation, therefore, can be responsible for your late period.

Women who participate in physically intense athletic activities may experience delays in their menstrual cycle, or they may not get their period at all. Experts have several theories for why extreme physical activity disrupts the menstrual cycle, ranging from a low percentage of body fat to poor nutrition and rapid weight loss. The Mayo Clinic, for example, states that having an excessively low body weight, one that is about 10 percent under normal weight, interrupts many hormonal functions and can interrupt ovulation. These same theories may help to explain why certain medical conditions — like anorexia, obesity and PCOS — can lead to late periods. 

Certain medicines or medical treatments, including chemotherapy and radiation, can also delay menstruation. And some drugs — including barbiturates (sedatives), corticosteroids, oral contraceptives (birth control pills) and certain tranquilizers — have also been found to disrupt the menstrual cycle.

Medical professionals often ask when a woman's last menstrual period (LMP) was to help determine if there is a chance she may be pregnant. There is often confusion as to what this actually means. "Most woman think that last menstrual period (LMP) means 'last day' of last menstrual period when in fact it is the first day of the last menstrual period," said Ramirez.

Menopause

If a woman is between 45 and 51, a missed period may be a sign of menopause. Menopause begins when the body runs out of eggs and stops ovulating. Often, women continue to menstruate for many years after ovulation stops, but it periods may become unpredictable. 

Additional resources

   U.S. National Library of Medicine: Hormonal Regulation of the Menstrual Cycle U.S. National Library of Medicine: Endocrine Regulation of Menstruation National Center for Biotechnology Information: Recovery of Menstrual Cycle After Therapy for Anorexia Nervosa - See more at: http://www.livescience.com/54922-what-is-ovulation.html#sthash.dwHfiT9D.dpuf

- See more at: http://www.livescience.com/54922-what-is-ovulation.html#sthash.dwHfiT9D.dpuf