A human egg survives for 12 to 24 hours after ovulation. That narrow window is the only time fertilization can occur, making it one of the shortest-lived cells involved in reproduction. Sperm, by comparison, can survive up to five days inside the reproductive tract, which is why the practical fertile window is much longer than the egg’s lifespan alone.
The 12-to-24-Hour Window
Once the ovary releases an egg, it travels into the fallopian tube and remains viable for roughly 12 to 24 hours. If sperm are present during that time, fertilization can happen. If not, the egg begins to break down and is eventually absorbed by the body or shed with the uterine lining during your next period.
Most of the egg’s fertilization potential is concentrated in the earlier part of that window. By the time you reach the tail end of 24 hours, the egg has already begun deteriorating, and the odds of successful fertilization drop sharply. Data from the British Fertility Society illustrates this well: the chance of pregnancy from intercourse two days before ovulation is about 26%, but it falls to just 1% if intercourse happens the day after ovulation. That steep decline reflects how quickly the egg loses viability once it’s released.
Why the Fertile Window Is Longer Than 24 Hours
If the egg only lasts a day, you might wonder why fertility guides talk about a five- or six-day fertile window. The answer is sperm survival. Sperm can live inside the female reproductive tract for up to five days, waiting in the fallopian tubes for an egg to arrive. So intercourse that happens several days before ovulation can still result in pregnancy if those sperm are still functional when the egg is released.
The American Society for Reproductive Medicine defines the fertile window as the six-day interval ending on the day of ovulation. That means the days with the highest conception probability are the five days leading up to ovulation and ovulation day itself. Once ovulation has passed and the egg’s 12-to-24-hour clock runs out, the fertile window closes until the next cycle.
How Your Body Signals the Window Is Closing
Your body gives visible clues that the egg’s window has ended. The most reliable one is a change in cervical mucus. In the days leading up to ovulation, rising estrogen makes cervical mucus thin, slippery, and stretchy, often compared to raw egg whites. This type of mucus helps sperm travel toward the egg.
After ovulation, progesterone takes over. This hormone thickens cervical mucus, making it tacky or dry. If you notice your mucus shift from wet and stretchy to thick and sticky, ovulation has likely passed, and the egg is either already fertilized or no longer viable. This drier pattern typically continues for the rest of the cycle until menstruation begins.
Progesterone levels continue rising for about five days after ovulation before dropping back down. That post-ovulation progesterone surge also raises your basal body temperature slightly, which is why temperature tracking can confirm that ovulation occurred, though it can’t predict it in advance.
What This Means for Timing
Because the egg’s lifespan is so short, timing intercourse before ovulation is more effective than trying to time it after. Sperm that are already waiting in the fallopian tubes when the egg arrives have the best chance of fertilizing it. Trying to conceive after ovulation has already happened leaves very little room for error, since the egg may have only hours of viability remaining by the time sperm reach it.
For people trying to conceive, the most productive approach is having intercourse in the two to three days before expected ovulation. Ovulation predictor kits, which detect a hormone surge that happens about 24 to 36 hours before ovulation, can help you identify that window in real time. Cervical mucus tracking adds another layer of information, since the appearance of wet, stretchy mucus signals that ovulation is approaching.
For people trying to avoid pregnancy, the takeaway is the same but reversed. The egg’s short survival means the post-ovulation phase of your cycle is naturally infertile, but only once you can confirm ovulation has actually happened. Relying on calendar estimates alone is unreliable, since ovulation timing can shift from cycle to cycle by several days.
Egg Quality Within the Window
Not all 12 to 24 hours are equal. The egg is at its healthiest immediately after release, and its quality declines as the hours pass. Eggs fertilized later in the window are more likely to have chromosomal issues, which can lead to failed implantation or early miscarriage. This is one reason why having sperm already in place before ovulation, rather than racing to catch an aging egg, improves the odds of a healthy conception.
Age also plays a role in egg quality independent of the survival window. The 12-to-24-hour lifespan stays roughly the same regardless of age, but the overall quality of the eggs released declines over time, particularly after the mid-30s. So while the clock on any individual egg doesn’t change, the starting condition of that egg does.