A human egg is viable for 12 to 24 hours after ovulation. That window is measured in hours, not days, making it one of the shortest fertility factors in the conception equation. Understanding exactly when this window opens and closes can help you time intercourse more effectively, whether you’re trying to conceive or trying to avoid pregnancy.
Why the Window Is So Short
Once the ovary releases an egg, it travels into the fallopian tube and begins to degrade almost immediately. The egg has no ability to renew itself or pause its biological clock. Within 12 hours it’s still in good shape for fertilization. By 24 hours, the egg has typically broken down to the point where a sperm can no longer penetrate and fertilize it successfully. After that, the unfertilized egg dissolves and is absorbed by the body or shed with the uterine lining during your next period.
This stands in sharp contrast to sperm, which survive 3 to 5 days inside the cervix, uterus, and fallopian tubes. That difference is the entire basis of the “fertile window” concept: sperm can arrive days early and wait for the egg, but the egg won’t wait for sperm.
Your Best Odds Are Before Ovulation, Not After
Because sperm outlast the egg by such a wide margin, the highest conception rates come from intercourse in the days leading up to ovulation rather than after it. The chance of pregnancy when sex occurs two days before ovulation is roughly 26%. That number drops to just 1% if sex happens one day after ovulation, according to data from the British Fertility Society. By that point, the egg is already nearing the end of its viable hours or has already passed them.
The practical takeaway: the three days before ovulation are your most fertile days. The day of ovulation itself still carries good odds because sperm can reach the egg within minutes to hours. But waiting until after ovulation leaves almost no margin. If you’re trying to conceive, having intercourse before you confirm ovulation gives sperm time to be in position when the egg arrives.
How to Tell When Ovulation Happens
Pinpointing ovulation precisely is tricky because the egg’s release isn’t something you can feel with certainty. But two tools give you useful estimates: ovulation predictor kits (OPKs) and basal body temperature tracking.
Ovulation Predictor Kits
OPKs detect a surge in luteinizing hormone (LH) in your urine. This surge happens roughly 24 to 48 hours before ovulation. After LH peaks, the egg is typically released within 8 to 20 hours. So a positive OPK means ovulation is likely 12 to 48 hours away. That’s your signal to have intercourse soon, ideally that day and the next, so sperm are already present when the egg drops.
Basal Body Temperature
Your resting body temperature rises slightly after ovulation, typically by 0.4°F to 1°F. You track this by taking your temperature first thing every morning before getting out of bed. The catch is that the rise confirms ovulation has already happened. By the time you see the shift, the egg is already into its 12-to-24-hour countdown or possibly past it. Basal body temperature is more useful for learning your cycle patterns over several months than for catching a specific egg in real time.
Cervical Mucus Changes
In the days leading up to ovulation, cervical mucus becomes clear, slippery, and stretchy, often compared to raw egg whites. This type of mucus helps sperm travel through the reproductive tract efficiently. After ovulation, rising progesterone levels cause the mucus to thicken and dry up. If you notice your cervical mucus has gone from slippery to sticky or dry, ovulation has likely already passed and the egg’s viable window is closing or closed.
What Happens if Fertilization Is Late
Fertilization that occurs toward the tail end of the egg’s viability raises some concerns. An aging egg is more prone to errors during cell division, which can lead to chromosomal abnormalities in the resulting embryo. In many cases, these abnormalities prevent implantation entirely, so a pregnancy never begins. In other cases, they may result in an early miscarriage. This is one reason fertility specialists emphasize timing intercourse to the days before ovulation: it gives the freshest possible egg the best chance of being fertilized by sperm that are already waiting.
Putting It All Together
The fertile window for any given cycle spans about five to six days: the five days before ovulation plus the day of ovulation itself. But the egg’s own contribution to that window is tiny, just 12 to 24 hours. Everything else in that fertile window relies on sperm longevity.
If you’re tracking your cycle to conceive, the most effective strategy is to have intercourse every one to two days during the five days leading up to expected ovulation. Relying on a single attempt after confirmed ovulation is a gamble against a very short clock. And if you’re using fertility awareness to avoid pregnancy, treat the egg’s 24-hour maximum as a hard boundary. Any unprotected intercourse in the days before ovulation carries significant risk because sperm will still be alive when the egg arrives.