After ovulation, an egg survives for only 12 to 24 hours. This is a surprisingly short window compared to sperm, which can live inside the reproductive tract for up to a week. That asymmetry is why the timing of intercourse relative to ovulation matters so much for both conception and contraception.
Why the Window Is So Short
Once the ovary releases an egg, it travels into the fallopian tube and begins to deteriorate almost immediately. The egg doesn’t divide, grow, or sustain itself the way sperm cells do. It has a fixed energy supply and a thin outer layer that becomes harder to penetrate as hours pass. By around 12 hours post-ovulation, fertilization rates start dropping sharply. By 24 hours, the egg is no longer viable.
This stands in stark contrast to sperm, which can survive in cervical mucus for an average of two to three days, and potentially up to a full week. That’s why sex before ovulation is far more likely to result in pregnancy than sex after it.
How Conception Odds Change by the Hour
The highest chance of pregnancy comes from intercourse in the one to two days before ovulation, not the day after. Data from the British Fertility Society illustrates this clearly: sex two days before ovulation carries roughly a 26% chance of conception per cycle, while sex one day after ovulation drops that to about 1%.
That 1% isn’t zero because there’s some natural variability in exactly when ovulation occurs, and some eggs may remain fertilizable slightly longer than average. But as a general rule, once you’re confident ovulation has already happened, the fertile window for that cycle is effectively closed.
The Fertile Window in Practice
Because sperm outlast the egg by such a wide margin, the practical fertile window stretches to about five or six days: the five days before ovulation plus the day of ovulation itself. If sperm are already waiting in the fallopian tube when the egg arrives, fertilization can happen within minutes to hours of the egg’s release. If sperm arrive late, they’re racing against a 12-to-24-hour clock that’s already ticking.
This is why fertility advice typically emphasizes having intercourse in the days leading up to ovulation rather than trying to time it precisely to the moment the egg is released. Waiting until after a confirmed temperature shift or other signs of completed ovulation usually means the opportunity has passed.
Timing Ovulation With LH Tests
Home ovulation predictor kits detect a surge in luteinizing hormone (LH) in your urine. A positive result means ovulation is likely within 12 to 48 hours, though the actual release of the egg tends to happen about 8 to 20 hours after the LH peak. The gap between detecting the surge and the peak itself adds some uncertainty, which is why a positive test is best treated as a signal to act soon rather than a precise countdown.
If you’re trying to conceive, the most effective approach is to have intercourse the day you get a positive LH test and the following day. This places sperm in the reproductive tract right around the time the egg is released, maximizing the overlap between sperm survival and egg viability. If you’re trying to avoid pregnancy, the LH surge signals that unprotected sex carries the highest risk over the next 24 to 48 hours.
What Happens If the Egg Isn’t Fertilized
When fertilization doesn’t occur, the egg disintegrates and is absorbed by the body. You won’t notice this happening. Meanwhile, the structure left behind on the ovary, called the corpus luteum, continues producing progesterone to maintain the uterine lining. About 10 days after ovulation, the corpus luteum breaks down, progesterone levels fall, and the lining sheds as a menstrual period. This typically puts your period at roughly 12 to 14 days after ovulation.
If fertilization does occur, the corpus luteum keeps producing progesterone for about 12 weeks, sustaining the early pregnancy until the placenta takes over hormone production.
Factors That May Affect Egg Quality
While the 12-to-24-hour survival window is fairly consistent regardless of age or health, the quality of the egg within that window is not. Age is the single biggest factor. Eggs from women in their late 30s and 40s are more likely to have chromosomal abnormalities, which reduces the chance of successful fertilization and implantation even when timing is perfect. Lifestyle factors like smoking and heavy alcohol use can also affect egg quality over time, though they don’t meaningfully change how long the egg survives after release.
The short lifespan of the egg is a fixed biological constraint. You can’t extend it with supplements, medications, or lifestyle changes. What you can influence is how well you time intercourse around ovulation, which is where tracking methods like LH testing, basal body temperature, and cervical mucus monitoring become useful tools.