Understanding your fertile window and ovulation

Ovulation usually happens 10 to 16 days before your next period starts, according to the NHS, and calculators use 14 days as the midpoint. The fertile window is counted as the day of ovulation plus the 5 days before it: a 1995 study of 221 women trying to conceive found conception only happened after sex in that six-day window. So in a regular 28-day cycle ovulation is around day 14 and the most fertile days are roughly days 9 to 14. Cycle length varies, so treat any estimate as a guide rather than a guarantee.

Informational only, not medical advice. Reviewed by our editorial team against NHS and gov.uk guidance. Speak to your GP if you have questions about your fertility.

When ovulation happens

Ovulation is when an ovary releases an egg. In most women it happens around 10 to 16 days before the next period (NHS), because the luteal phase, from ovulation to the period, varies less than the first half of the cycle. That is why ovulation is usually estimated as your cycle length minus 14: in a 28-day cycle it falls near day 14, and in a 32-day cycle nearer day 18.

The 6-day fertile window

An egg survives for only about a day after release, but sperm last longer: the NHS says up to 7 days in the fallopian tubes. In practice, a 1995 study in the New England Journal of Medicine found conception only happened when sex took place in a six-day window ending on the day of ovulation, with the chance rising from about 10% five days before to about 33% on the day itself. That is why the fertile window is counted as the day of ovulation plus the 5 days before it.

Why cycle length matters

Because the estimate works back from your next period, your cycle length changes where the window lands. If your cycles are irregular, the calendar estimate is less reliable, and ovulation predictor kits or tracking your temperature and other signs can give more certainty. If you are trying to conceive and have questions, your GP can help.

Estimate your fertile window

The BabyData ovulation calculator applies these rules from the first day of your last period and your average cycle length, and shows your estimated ovulation day, most fertile days and next period. If you conceive, the same page estimates your due date.

Sources

• NHS: periods and fertility in the menstrual cycle (timing of ovulation, sperm survival).
• Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995 (the six-day fertile window).