Your most fertile window
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Estimated ovulation
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Next period due
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Find your most fertile days and estimated ovulation date.
Ovulation is when an ovary releases an egg, and it typically happens about 14 days before your next period starts, not 14 days after the last one began, an important distinction if your cycle is not exactly 28 days. The calculator above estimates your ovulation day and fertile window from your cycle length and last period, which is why an accurate cycle length gives a better result.
The fertile window is a few days wide. Because sperm can survive several days and the egg lives about a day, the days most likely to lead to conception are roughly the five days before ovulation plus the day itself. Timing around this window, rather than a single day, is what matters when trying to conceive.
Calendar estimates are a starting point. Cycle length varies month to month, so a calendar prediction is an estimate, not a guarantee. Tracking basal body temperature (which rises slightly after ovulation) or using ovulation predictor kits (which detect the hormone surge before it) pinpoints the day more reliably, especially for irregular cycles.