Ovulation is the moment when one of your ovaries releases a mature egg into the fallopian tube, where it can potentially be fertilized by sperm. It happens once per menstrual cycle, typically around the midpoint, and the released egg survives for roughly 17 hours. That narrow window makes ovulation the central event in fertility, and understanding how it works gives you a clearer picture of your cycle, your body’s signals, and your chances of conception.
How Ovulation Works
Ovulation is the result of a carefully timed hormonal chain reaction that begins in your brain. The hypothalamus, a small region at the base of your brain, sends out a signaling hormone in rhythmic pulses. Slow pulses trigger the pituitary gland to release a hormone called FSH, which tells the ovaries to start maturing a batch of follicles, each containing an immature egg. Over the first half of your cycle, one follicle outgrows the others and becomes dominant.
As that dominant follicle matures, it produces rising levels of estrogen. When estrogen reaches a critical threshold, the brain’s signaling pulses speed up, and the pituitary gland responds with a sudden surge of luteinizing hormone (LH). This LH surge is the direct trigger for ovulation. It activates enzymes that weaken and break down the wall of the follicle, allowing the mature egg to burst free from the ovary’s surface. The fimbriae, finger-like projections at the end of the fallopian tube, sweep the egg inside.
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. The corpus luteum begins producing progesterone, a hormone that thickens the uterine lining in preparation for a possible pregnancy. If fertilization doesn’t happen, the corpus luteum breaks down about two weeks later, progesterone drops, and your period begins.
When Ovulation Happens in Your Cycle
A common assumption is that ovulation falls neatly on day 14 of a 28-day cycle, but real cycles are far more variable. The cycle has two main phases: the follicular phase (before ovulation) and the luteal phase (after ovulation). In a large population study, the median follicular phase length was 16.5 days, with a wide range of 10 to 28 days depending on the person and the cycle. The luteal phase was more consistent, with a median of about 12 days and much less variation.
This means that the follicular phase is the wildcard. Stress, illness, travel, or simply natural variation can delay ovulation by days or even weeks, which shifts your entire cycle longer. The luteal phase, by contrast, stays relatively stable from cycle to cycle. Age also plays a role: women in their 20s tend to have longer follicular phases (averaging 18 days) compared to women in their 40s (averaging about 15 days), while the luteal phase length stays essentially the same across age groups.
The Fertile Window
Because the egg survives for less than a day after ovulation, the fertile window depends heavily on sperm survival. Sperm can live in the reproductive tract for an average of about 1.4 days, though some sperm have a 5% chance of surviving more than 4 days and a small chance of lasting nearly a week. This means pregnancy is possible from intercourse that happens several days before ovulation, not just on the day itself. The most fertile days are the two to three days leading up to ovulation and the day of ovulation.
Physical Signs of Ovulation
Your body offers several clues that ovulation is approaching or has occurred, though not everyone notices them.
Cervical mucus changes. In the days before ovulation, cervical mucus becomes clear, slippery, and stretchy, often compared to raw egg whites. This consistency helps sperm travel through the cervix more easily. You typically get this type of mucus for about three to four days. After ovulation, mucus becomes thicker, stickier, or dries up entirely.
Ovulation pain. About one in five women feel a twinge or cramp on one side of their lower abdomen around the time of ovulation. This is sometimes called mittelschmerz (German for “middle pain”). It can last anywhere from a few minutes to 24 or 48 hours and may switch sides from cycle to cycle, depending on which ovary releases the egg.
Basal body temperature shift. After ovulation, the progesterone produced by the corpus luteum causes your resting body temperature to rise slightly, typically by 0.4 to 1.0 degree Fahrenheit. This shift only confirms that ovulation has already happened, so it’s useful for pattern tracking over multiple cycles rather than predicting ovulation in real time.
How Ovulation Is Detected and Confirmed
Home ovulation predictor kits (OPKs) work by detecting the LH surge in your urine. Research suggests that a urinary LH threshold of 25 to 30 mIU/mL is the best cutoff for predicting ovulation within 24 hours, with a 98% reliability for ruling ovulation out when the test is negative. A positive result is less definitive on its own, correctly predicting ovulation about 50 to 60% of the time, since LH can surge without a follicle successfully releasing an egg.
Ultrasound monitoring, done in a clinic, allows a doctor to directly watch follicle growth and confirm the egg’s release. A prospective study comparing the two approaches in couples trying to conceive found that women using ultrasound-guided monitoring had roughly twice the odds of becoming pregnant compared to those using home ovulation tests alone. Ultrasound is more involved and expensive, so it’s typically reserved for people working with a fertility specialist.
Blood progesterone levels offer another way to confirm ovulation after the fact. A progesterone level above 2.3 ng/mL, measured a few days after suspected ovulation, is a strong indicator that ovulation has occurred. Levels above 3.0 ng/mL confirm it with high certainty and indicate the start of the naturally infertile portion of the cycle.
When Ovulation Doesn’t Happen
A cycle without ovulation is called an anovulatory cycle. You can still have what looks like a period, because hormone levels may fluctuate enough to build and shed some uterine lining, but no egg is released. Occasional anovulatory cycles are normal at certain life stages, particularly in the first few years after periods begin and in the years leading up to menopause.
Chronic anovulation, where ovulation is consistently absent or irregular, has a range of causes. Polycystic ovary syndrome (PCOS) is the most common, involving hormonal imbalances that prevent follicles from maturing fully. Thyroid disorders, particularly an underactive thyroid, can disrupt the hormonal signals needed for ovulation. Problems with the pituitary gland, which controls LH and FSH production, can also shut down the process.
Lifestyle factors play a significant role as well. Very low body weight, often related to eating disorders or excessive exercise, can cause the brain to reduce or stop its hormonal signaling to the ovaries. Obesity can have a similar effect through different hormonal pathways. High levels of prolonged stress can delay or suppress ovulation by disrupting the same brain signals. Certain medications, including anabolic steroids and some psychiatric drugs, can interfere with ovulation too.
Breastfeeding is another common and temporary cause. The hormone prolactin, which drives milk production, suppresses the signals that trigger ovulation. This effect varies widely: some breastfeeding women resume ovulating within weeks, while others don’t ovulate for months.