A human egg survives for 12 to 24 hours after ovulation. That’s a remarkably short window compared to sperm, which can live inside the reproductive tract for three to five days. The highest pregnancy rates occur when sperm and egg meet within four to six hours of the egg’s release, so timing matters more than most people realize.
Why the Window Is So Short
Once the ovary releases an egg, it enters the fallopian tube and begins a slow journey toward the uterus. Fertilization needs to happen in the fallopian tube itself, and the egg’s outer layer starts to degrade relatively quickly. After about 12 hours, the egg’s ability to be fertilized drops significantly. By 24 hours, the window is effectively closed.
This is a sharp contrast with sperm. Sperm cells can survive in the cervix, uterus, and fallopian tubes for up to five days, essentially waiting for the egg to arrive. That asymmetry is why sex before ovulation is often more effective for conception than sex after it.
The Fertile Window Is Wider Than You’d Think
Even though the egg only lasts about a day, the overall fertile window spans roughly six days per cycle. The American College of Obstetricians and Gynecologists defines it this way: you can get pregnant from sex up to five days before ovulation (because sperm survive that long) or up to one day after ovulation (before the egg breaks down). That six-day stretch is your realistic conception window each month.
The most fertile days within that range are the two to three days leading up to ovulation and the day of ovulation itself. Having sex on those days gives sperm time to reach the fallopian tube and be ready when the egg appears. Waiting until after ovulation to have sex for the first time that cycle leaves very little margin.
How to Know When Ovulation Happens
Pinpointing ovulation is the practical challenge. Your body produces a surge of luteinizing hormone (LH) roughly 24 to 48 hours before the egg is released. Over-the-counter ovulation predictor kits detect this surge in your urine. A positive result means ovulation is likely within 12 to 48 hours, with most people ovulating about 8 to 20 hours after LH reaches its peak.
Basal body temperature offers a different kind of signal. After ovulation, your resting temperature rises by roughly 0.4 to 1.0°F and stays elevated for the rest of that cycle. The catch is that this shift confirms ovulation has already happened, so by the time you see it, most of the egg’s viable hours may have passed. Temperature tracking is more useful for confirming your cycle patterns over several months than for timing sex in real time.
Timing Sex for Conception
Because the egg deteriorates quickly, the best strategy is to have sperm already in the fallopian tubes before the egg arrives. That means having sex in the days leading up to ovulation rather than trying to react to it. If you get a positive ovulation test, that day and the following day are your highest-priority windows, since the egg will likely appear within the next one to two days.
For couples trying to conceive, having sex every one to two days during the fertile window covers the timing well without needing to pinpoint the exact hour of ovulation. The combination of sperm’s multi-day survival and the egg’s brief viability means that frequency and general timing matter more than precision.
What Happens if the Egg Isn’t Fertilized
If sperm don’t reach the egg within that 12-to-24-hour window, the egg continues traveling down the fallopian tube and disintegrates. It’s reabsorbed by the body, and hormone levels shift to prepare the uterine lining for shedding. Menstruation typically follows about 14 days after ovulation, marking the start of the next cycle. The unfertilized egg itself is microscopic, so there’s nothing you’d notice physically when it breaks down.