An egg survives for about 12 to 24 hours after ovulation. Once released from the ovary into the fallopian tube, it has a narrow window in which it can be fertilized by sperm. After that, it begins to break down and is eventually absorbed by the body.
That short lifespan is why timing matters so much for conception, and why understanding the hours around ovulation can make a real difference whether you’re trying to get pregnant or trying to avoid it.
The 12-to-24-Hour Window
When your ovary releases an egg, it enters the fallopian tube and begins its slow journey toward the uterus. During this transit, the egg is capable of being fertilized for roughly 12 to 24 hours. The American College of Obstetricians and Gynecologists places the upper limit at 24 hours, while some reproductive health centers narrow the peak viability to 12 to 24 hours. In practical terms, the egg is at its healthiest in the first several hours after release, with fertilization chances declining as the clock ticks toward that 24-hour mark.
If no sperm reaches the egg in time, it stops being viable, disintegrates, and is shed along with the uterine lining during your next period.
Why the Fertile Window Is Longer Than 24 Hours
Even though the egg itself lives less than a day, sperm can survive inside the reproductive tract for up to five days. This means sex that happens days before ovulation can still lead to pregnancy, because sperm may already be waiting in the fallopian tube when the egg arrives. ACOG estimates you can become pregnant if you have sex anywhere from five days before ovulation until one day after.
That creates a fertile window of roughly six days total. Your highest chance of conception comes when live sperm are already present in the fallopian tube at the moment the egg is released. In practice, the two or three days leading up to ovulation and the day of ovulation itself are the most fertile. The day after ovulation still carries some possibility, but it drops off sharply because the egg is already aging.
How Your Body Signals Ovulation
The egg’s countdown starts the moment it leaves the ovary, so knowing when ovulation happens helps you make sense of that 12-to-24-hour window. Your body gives a few trackable signals.
LH Surge
A spike in luteinizing hormone (LH) triggers the release of the egg about 36 to 40 hours later. This is what over-the-counter ovulation predictor kits detect. A positive test means ovulation is likely coming within the next day and a half or so, giving you a heads-up before the egg is even released. This is the most useful signal if you’re trying to time intercourse, because it comes before the window opens rather than after.
Basal Body Temperature
Your resting body temperature rises slightly after ovulation, typically by less than half a degree Fahrenheit (the increase can range from 0.4°F to 1°F depending on the person). When you see higher temperatures for at least three consecutive days, you can assume ovulation has already occurred. The limitation here is that by the time you confirm the temperature shift, the egg has likely already passed its viable window. Basal body temperature tracking is better for understanding your cycle patterns over several months than for catching the fertile window in real time.
Progesterone Rise
After ovulation, progesterone levels climb for about five days before dropping again. This hormonal shift prepares the uterine lining for a potential pregnancy. It also signals that the fertile phase is over. Some at-home hormone tests measure progesterone to confirm that ovulation took place, which is useful for cycle tracking but, like temperature, tells you after the fact.
What This Means for Getting Pregnant
Because the egg’s lifespan is so short, the most effective strategy for conception is having sperm already in place before ovulation occurs. Couples trying to conceive often have the best results when they have sex during the two to three days before the expected ovulation date. Waiting until after you think you’ve ovulated leaves very little margin, since the egg may already be nearing the end of its viable hours.
If you’re using ovulation predictor kits, having sex on the day of a positive result and the following day covers the window well. The positive kit tells you the LH surge has happened, and ovulation will follow within roughly 36 to 40 hours. That timing puts sperm in the fallopian tube right as the egg arrives.
What This Means for Avoiding Pregnancy
If you’re using fertility awareness to prevent pregnancy, the egg’s short lifespan can be misleading. It’s tempting to think you only need to avoid unprotected sex for one day, but sperm survival extends the risk zone to five or six days. Because ovulation timing can shift from cycle to cycle, most fertility awareness methods build in buffer days on either side of the expected ovulation date. Relying on a single method of detection, especially basal body temperature alone, leaves room for error since it only confirms ovulation after it has already happened.
The asymmetry between sperm lifespan (up to five days) and egg lifespan (one day at most) is the central fact of the fertile window. Sperm are the variable that stretches the window earlier in the cycle, while the egg’s rapid decline closes it sharply on the back end.