Once an egg is released from the ovary, it can be fertilized for roughly 12 to 24 hours. That window is shorter than most people expect. The highest pregnancy rates occur when sperm meets the egg within 4 to 6 hours of ovulation, and the chances drop sharply after that. By 24 hours post-release, the egg is no longer viable.
Why the Window Is So Short
After the ovary releases an egg, it enters the fallopian tube and travels toward the uterus. That journey through the tube takes about 30 hours. Along the way, the egg pauses at a specific junction in the tube and rests there for roughly another 30 hours. This resting spot is where fertilization happens, assuming sperm are present.
The egg doesn’t stay fertile for that entire transit, though. Its outer layer begins to degrade within hours. If no sperm reaches it in time, the egg breaks down and is reabsorbed by the body or shed during your next period. There’s no way to extend this window through diet, supplements, or lifestyle changes. It’s a fixed biological constraint.
Timing Sex Around Ovulation
Because the egg’s lifespan is so brief, the days leading up to ovulation matter more than the day after. Sperm can survive inside the cervix, uterus, and fallopian tubes for 3 to 5 days. That means sperm deposited days before ovulation can still be waiting in the fallopian tube when the egg arrives.
Data from the British Fertility Society illustrates this clearly. The chance of pregnancy is highest when intercourse happens in the three days before ovulation. Sex two days before ovulation carries about a 26% chance of conception per cycle. Sex one day after ovulation drops that to roughly 1%. The practical takeaway: sperm should already be in the reproductive tract when the egg is released, not racing to catch up afterward.
The Fertile Window Is About 6 Days
Combining sperm survival (up to 5 days) with the egg’s short lifespan (under 24 hours) gives you a fertile window of about 6 days per cycle. That window opens approximately 5 days before ovulation and closes the day of ovulation itself. While conception the day after ovulation is technically possible, the odds are extremely low.
This is why timing matters so much for couples trying to conceive. Having sex every day or every other day during the five days before and the day of ovulation covers the window effectively. Waiting until you’re sure you’ve ovulated usually means the best opportunity has already passed.
How to Tell When You’re Ovulating
Ovulation happens about 36 to 40 hours after a surge in luteinizing hormone (LH). Home ovulation predictor kits detect this surge in urine, giving you a one- to two-day heads-up before the egg is actually released. A positive test means ovulation is approaching, not that it’s already happened.
Cervical mucus offers another signal. In the days before ovulation, discharge becomes wet, stretchy, and slippery, often compared to raw egg whites. This type of mucus helps sperm survive and travel through the reproductive tract. After ovulation, mucus dries up and becomes thick and sticky again as progesterone takes over the cycle. If you notice that shift from slippery to dry, ovulation has likely already occurred.
Basal body temperature tracking works too, but it confirms ovulation after the fact. Your resting temperature rises slightly (about 0.5°F) after the egg is released and stays elevated for the rest of the cycle. By the time you see the temperature shift, the fertile window is closing or already closed. Combining mucus observation with LH testing gives you the most useful advance notice.
What Happens After Fertilization
If a sperm does reach the egg in time, the egg’s surface changes almost instantly. Tiny granules just beneath the egg’s outer membrane release their contents, chemically altering the egg’s coating so no additional sperm can penetrate. This reaction prevents the egg from being fertilized by more than one sperm.
The fertilized egg, now called a zygote, continues its journey down the fallopian tube. About a week after fertilization, it reaches the uterus and begins the process of implanting into the uterine lining. Implantation, not fertilization, is when pregnancy hormones start rising and when a pregnancy test will eventually turn positive.
Age and Egg Quality
The 12-to-24-hour fertilization window applies regardless of age, but egg quality changes over time. As women get older, the eggs released each cycle are more likely to have chromosomal irregularities, which makes successful fertilization and implantation less probable even when timing is perfect. This decline in quality happens gradually through the 30s and accelerates after 35. The window of viability doesn’t shrink in hours, but the likelihood that any given egg will result in a healthy pregnancy does decrease with age.