Ovulation is the release of a mature egg from an ovary. It usually occurs once during a menstrual cycle, and it marks the time when pregnancy is most likely to occur. The fertile window includes the several days before ovulation and the short period around and after it when sperm and the egg can still meet.
Ovulation is controlled by a coordinated rise and fall of reproductive hormones. As the cycle progresses, an ovarian follicle—a small fluid-filled structure containing an immature egg—grows and prepares the egg for release. Near ovulation, a surge in luteinizing hormone (LH) triggers the follicle to release the mature egg into the fallopian tube.
Understanding what happens during this period requires looking at both the timing of ovulation and how long sperm and the egg remain capable of participating in fertilization.
What happens during ovulation?
At the beginning of a menstrual cycle, several follicles in an ovary begin developing under the influence of hormones. Usually, one becomes the dominant follicle and continues growing while the others stop developing.
The developing follicle produces increasing amounts of estrogen. As estrogen levels rise, they eventually trigger a change in the brain and pituitary gland that produces a rapid increase in LH, known as the LH surge.
The LH surge causes a series of changes inside the dominant follicle. The follicle weakens at its surface and eventually ruptures, releasing the mature egg from the ovary. This is ovulation.
The egg is then swept into the nearby fallopian tube, where fertilization can occur if sperm are present.
Ovulation does not mean that fertilization has occurred. It simply means that an egg has been released and is available for possible fertilization.
When does ovulation occur?
Ovulation does not necessarily happen on the same calendar day for everyone. It also does not always occur on the same day of every person’s cycle.
A common misconception is that ovulation always happens on day 14 of a menstrual cycle. Day 14 is a useful example for a typical 28-day cycle, but menstrual cycles vary, and the timing of ovulation can vary with them.
Ovulation generally occurs about two weeks before the next menstrual period, rather than necessarily two weeks after the previous period began. Because the length of the first part of the cycle can vary considerably, the day of ovulation can shift from one cycle to another.
This is why counting backward from the expected next period can sometimes give a better estimate than assuming ovulation occurs on a fixed cycle day.
What is the fertile window?
The fertile window is the period when sexual intercourse can result in pregnancy because sperm may still be alive when an egg is released, or the egg may still be available for fertilization after ovulation.
The timing of the fertile window is determined largely by two different survival times. Sperm can remain capable of fertilization in the female reproductive tract for several days under favorable conditions. The egg, by contrast, remains capable of being fertilized for only a relatively short time after ovulation.
As a result, intercourse several days before ovulation can potentially lead to pregnancy. Sperm can already be present in the reproductive tract when the egg is released.
The highest likelihood of conception is generally around the days immediately preceding ovulation and the day of ovulation itself.
This also explains why the fertile window is broader than the brief moment when the egg leaves the ovary.
Why can pregnancy happen from sex before ovulation?
Pregnancy does not require sperm and an egg to arrive at exactly the same moment.
After ejaculation, sperm can move through the cervix and into the uterus and fallopian tubes. Some sperm can survive there for several days when conditions are suitable.
If ovulation occurs while viable sperm are still present, fertilization can occur. This means intercourse before ovulation can result in pregnancy even though the egg had not yet been released at the time of intercourse.
This is one reason predicting a single “ovulation day” is less useful for understanding fertility than considering the entire fertile window.
What happens to the egg after ovulation?
After the follicle releases the egg, the egg enters the fallopian tube. Tiny structures and muscular movements within the reproductive tract help move it through the tube.
If viable sperm reach the egg and one successfully penetrates it, fertilization occurs. The resulting cell begins dividing as it travels toward the uterus.
If fertilization does not occur during the egg’s limited period of viability, the egg breaks down and is absorbed.
Meanwhile, the follicle that released the egg changes into a temporary hormone-producing structure called the corpus luteum. It produces progesterone, a hormone that helps prepare and maintain the uterine lining for a possible pregnancy.
If pregnancy does not occur, the corpus luteum eventually breaks down. Hormone levels fall, and the uterine lining is shed during the next menstrual period.
What signs can occur around ovulation?
Some people notice physical changes around ovulation, while others notice little or nothing.
One common change is in cervical mucus, the fluid produced by glands in the cervix. As ovulation approaches, rising estrogen can make this mucus clearer, wetter, more slippery, and more stretchy. These changes can create conditions that help sperm move through the reproductive tract.
Some people also experience mild pelvic or lower-abdominal discomfort around ovulation. This is sometimes called mittelschmerz, a German term meaning “middle pain” because it can occur around the middle of a menstrual cycle.
Other possible changes include breast tenderness, bloating, or changes in sexual desire. These symptoms are not specific enough by themselves to confirm that ovulation has occurred.
How do ovulation tests detect the fertile period?
Ovulation predictor tests commonly detect luteinizing hormone in urine. Because the LH surge occurs shortly before ovulation, detecting a rise in LH can provide an indication that ovulation may be approaching.
An LH test does not directly show that an egg has been released. It detects the hormonal signal associated with the process.
This distinction matters because hormone patterns can vary, and detecting an LH surge does not guarantee that ovulation will definitely occur.
Another way to assess the menstrual cycle is by tracking basal body temperature, which is the body’s resting temperature measured under consistent conditions. Progesterone released after ovulation causes a small rise in basal body temperature. Because this change occurs after ovulation, temperature tracking is generally more useful for identifying that ovulation has already happened than for predicting it in advance.
Changes in cervical mucus can provide another clue that ovulation is approaching.
What happens to hormones during the fertile window?
The fertile window is the result of a changing hormonal sequence rather than a single hormone acting alone.
Early in the cycle, follicle-stimulating hormone (FSH) helps stimulate the development of ovarian follicles. As a dominant follicle develops, it produces increasing amounts of estrogen.
Rising estrogen eventually contributes to the LH surge. The surge triggers the final steps leading to ovulation.
After ovulation, the corpus luteum produces progesterone. Progesterone changes the uterine lining and also influences cervical mucus and body temperature.
The changing levels of estrogen, LH, FSH, and progesterone coordinate the ovarian and uterine events that make pregnancy possible.
Does ovulation always happen once per cycle?
Usually, one ovulation event occurs during a menstrual cycle, but biology is not perfectly uniform.
In some cycles, more than one egg can be released. If multiple eggs are fertilized, this can result in fraternal twins or other multiple pregnancies.
Releasing multiple eggs does not necessarily mean ovulation occurs on widely separated days. The eggs can be released during the same general ovulatory period.
It is also possible for a cycle to occur without ovulation. Such a cycle is called anovulatory, meaning that no egg is released.
Why the fertile window can be difficult to predict
The timing of ovulation depends on the interaction of several hormones and can change from cycle to cycle. Stress, illness, changes in sleep, major changes in weight or exercise, certain medications, breastfeeding, and hormonal conditions can affect menstrual patterns and ovulation.
For this reason, a calendar calculation alone cannot reliably identify the fertile window for every person.
The important biological point is that fertility is concentrated around ovulation because the egg has a short lifespan, while sperm can remain viable for substantially longer under favorable conditions. The days leading up to ovulation therefore matter as much as, and often more than, the period immediately after it.
Ovulation is ultimately a precisely coordinated reproductive event: a developing follicle matures an egg, hormonal signals trigger its release, the egg enters the fallopian tube, and a short period follows during which fertilization is possible.
