For a typical 28-day cycle, you’re most fertile starting around day 9 or 10, which is roughly 3 to 5 days after your period ends. But that number shifts depending on how long your cycle actually is, how long your period lasts, and whether you ovulate earlier or later than average. The fertile window itself spans about seven days: the five days before ovulation, the day of ovulation, and the day after.
Why the Answer Depends on Your Cycle
Ovulation typically happens about 12 to 14 days before your next period starts. For someone with a textbook 28-day cycle, that puts ovulation around day 14 (counting the first day of your period as day 1). If your period lasts five days, that means you’d enter your fertile window roughly four or five days after bleeding stops.
But cycles vary. If your cycle runs 25 days, you likely ovulate around day 11, which means your fertile window could open as early as day 6, possibly while you’re still bleeding. If your cycle is 32 days, ovulation may not happen until day 18 or later, pushing your most fertile days further out. The first half of your cycle, called the follicular phase, is the part that fluctuates. It can range from 14 to 21 days, depending on how quickly your body prepares an egg. The second half stays relatively consistent at around 14 days.
This variability matters more than most people realize. A large population study from Norway found that more than a third of women with regular, normal-length cycles didn’t ovulate in a given month at all. Ovulation isn’t as clockwork-predictable as the “day 14” rule suggests.
How the Fertile Window Actually Works
Your fertile window exists because of two biological timers. Sperm can survive inside the reproductive tract for up to five days, while a released egg lives for less than 24 hours. The highest chances of conception occur when sperm are already waiting in the fallopian tube when the egg arrives. Pregnancy rates peak when sperm and egg meet within four to six hours of ovulation.
This is why the days before ovulation matter more than the day after. Having sex two or three days before you ovulate gives sperm time to travel and be in position. The day after ovulation, the window is already closing.
Calculating Your Personal Fertile Window
If your cycles are consistent, a simple formula can estimate your window. Track your cycle lengths for six months, then take your shortest cycle and subtract 18, and take your longest cycle and subtract 11. Those two numbers give you the range of your fertile days. For example, if your shortest cycle is 27 days and your longest is 32, your fertile window falls between day 9 and day 21 of your cycle.
That’s a wide range, and it’s meant to be conservative. If you’re trying to conceive, focusing on the narrower window of five to seven days around your estimated ovulation date is more practical. If you’re trying to avoid pregnancy, the wider range accounts for the months your body does something unexpected.
Physical Signs That Fertility Is Peaking
Your body gives real-time signals that are often more reliable than calendar math alone. The most useful one is cervical mucus. In the days leading up to ovulation, discharge changes from thick, sticky, and white to transparent, stretchy, and slippery, similar to raw egg white. That egg-white consistency signals your most fertile days. When discharge feels wet and slippery, ovulation is close.
Your body also releases a surge of luteinizing hormone (LH) about 36 to 40 hours before the egg is released. This is what over-the-counter ovulation predictor kits detect. A positive result means you’re likely to ovulate within the next day or two, marking your peak fertility.
Basal body temperature, your resting temperature first thing in the morning, offers a different kind of signal. After ovulation, your temperature rises by about half a degree Fahrenheit and stays elevated. When that rise holds steady for three days, ovulation has passed and the fertile window is closed. The catch is that temperature only confirms ovulation after the fact. It’s useful for learning your pattern over several months but won’t tell you in advance when to time intercourse this cycle.
How Age Changes the Timeline
As you move through your late 30s and into your 40s, cycles often get shorter. This happens because the follicular phase compresses. Your body requires stronger hormonal signals to mature an egg, and cycles that once ran 28 to 30 days may shrink to 21 to 25 days. That means ovulation happens earlier, and the fertile window shifts closer to your period.
Eventually, cycles may become irregular as ovulation stops happening consistently. If your cycle length has changed noticeably, recalculating your fertile window based on your current pattern rather than relying on old assumptions is important.
Putting It All Together
For most people, the most fertile days fall between day 10 and day 16 of a 28-day cycle, or roughly 5 to 10 days after bleeding starts. But that estimate is only as good as your cycle is regular. Combining calendar tracking with cervical mucus observation and, if helpful, ovulation test strips gives you a much clearer picture than any single method alone. The goal is to identify the two or three days just before ovulation, when the odds of conception are highest.