Ovulation Window

The ovulation window is the span of days in a menstrual cycle when conception is possible, built around the single day an egg is released from the ovary. It's wider than just ovulation day itself because sperm can survive inside the reproductive tract for several days, meaning intercourse before ovulation can still result in conception once the egg is released.

Cycle tracking apps and calculators estimate this window from your average cycle length and the start date of your last period. Get a personalised estimate with the Ovulation Calculator, or the Clomid Ovulation Calculator if you're tracking a medicated cycle.

Estimated Ovulation Day = Cycle Length − 14 (counted from the first day of your last period)

Fertile Window = (Ovulation Day − 5) to (Ovulation Day + 1)

This assumes a standard 14-day luteal phase (the time between ovulation and the next period), which holds fairly consistently even when the overall cycle length varies.

For a 30-day cycle, estimated ovulation falls on day 16 (30 − 14). The fertile window runs from day 11 through day 17, giving a 7-day span centred on the estimated ovulation date.

• The luteal phase is more consistent than the follicular phase. Most of the variation in cycle length happens before ovulation, not after, which is why the "cycle length minus 14" formula works reasonably well even for cycles that aren't exactly 28 days.
• Timing intercourse for the days before ovulation, not just on it, matters more than people expect given how long sperm can survive compared to the egg's short viability window.
• Irregular cycles reduce the formula's accuracy. If your cycle length swings by more than a few days month to month, a single calendar estimate is less reliable than tracking physical signs over several cycles.
• Cycle tracking often gets paired with broader pregnancy planning tools; once conception occurs, Gestational Weight Gain and Preterm Birth become the next relevant concepts to understand.