A released egg survives for less than 24 hours after ovulation. Most fertility specialists put the realistic window for fertilization even shorter, closer to 12 to 18 hours. After that, the egg begins to deteriorate rapidly and can no longer be fertilized.
That narrow window is why the timing of intercourse relative to ovulation matters so much, and why the days *before* ovulation are actually more important than the day of ovulation itself.
What Happens to the Egg After Release
When an egg leaves the ovary, it enters the fallopian tube and begins traveling toward the uterus. Fertilization needs to happen during this journey, specifically in the upper portion of the fallopian tube. The clock starts immediately.
Within just a few hours, the egg’s internal structures start breaking down. The spindle, a scaffolding that holds chromosomes in place, begins stretching and detaching from the egg’s outer wall. Chromosomes can slip out of alignment in as little as four hours. By 24 hours, nearly all of these internal structures have come apart. At the same time, the egg ramps up a self-recycling process, breaking down its own components. Calcium signaling and stress-response pathways activate, essentially marking the egg for disposal.
Even if a deteriorating egg were somehow fertilized late in this process, the resulting embryo would have a high risk of chromosomal abnormalities. This is one reason fertility declines sharply on the day after ovulation.
Why the Days Before Ovulation Matter Most
Sperm can survive inside the reproductive tract for three to five days. The egg lasts less than one. This mismatch is the key to understanding your fertile window.
The American Society for Reproductive Medicine defines the fertile window as the six-day interval ending on the day of ovulation. In a study of 221 women, the highest chance of pregnancy occurred when intercourse happened within two days before ovulation. The likelihood of conception was already declining when intercourse took place on ovulation day itself, because by the time sperm reach the egg, much of its viable window may have passed.
In practical terms, sperm that are already waiting in the fallopian tube when the egg arrives have a much better shot at fertilization than sperm that need to make the trip after ovulation has occurred.
Pinpointing When Ovulation Happens
Knowing the egg’s short lifespan only helps if you can estimate when it’s released. The most common home method is testing for the LH surge, a spike in luteinizing hormone that triggers the egg’s release. The onset of this surge typically precedes ovulation by about 36 hours, while the peak comes roughly 10 to 12 hours before the egg is released.
So a positive ovulation test doesn’t mean you’re ovulating right now. It means ovulation is likely within the next day or so, which makes it a useful signal to time intercourse. By the time you’d get a second positive test or notice a temperature shift confirming ovulation has already happened, the egg’s window is largely closing.
It’s also worth noting that ovulation timing varies more than most people expect. A large prospective study found that only 54% of women were in their fertile window on cycle days 12 and 13, the days most commonly associated with ovulation. At least 10% of women with regular cycles were in their fertile window on any given day between cycle days 6 and 21. Women with shorter cycles sometimes reached their fertile window by the end of the first week.
What Happens After the Window Closes
Once the egg’s viable hours are up and fertilization hasn’t occurred, the egg disintegrates and is absorbed by the body. You won’t notice this happening. There’s no discharge or symptom associated with the egg breaking down.
Meanwhile, the follicle that released the egg transforms into a temporary structure called the corpus luteum, which forms on the day of ovulation itself. This structure produces progesterone, the hormone that prepares the uterine lining for a potential pregnancy. If no embryo implants, the corpus luteum breaks down about 10 to 14 days later, progesterone drops, and your period begins.
Putting the Timeline Together
Here’s how the key durations stack up:
- Sperm survival: 3 to 5 days inside the reproductive tract
- Egg survival: 12 to 24 hours after ovulation
- Fertile window: roughly 6 days, ending on ovulation day
- LH surge to ovulation: about 24 to 36 hours from the start of the surge
The overall chance of pregnancy in any single cycle is about 21%, even when timing is reasonable. The asymmetry between sperm and egg lifespan means the most effective strategy for conception is having intercourse in the days leading up to ovulation rather than trying to catch the egg after it’s released. Once ovulation has clearly passed, the fertile window for that cycle is closed.