Understanding your fertile window and ovulation

Ovulation usually happens about 14 days before your next period starts. The fertile window is the day of ovulation plus the 5 days before it, because sperm can survive in the body for up to about 5 days, as the NHS notes. So in a 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. It usually happens about 14 days before your next period, because the luteal phase, from ovulation to the period, stays roughly constant at around 14 days while the first half of the cycle varies. That is why ovulation is 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 lives for about 12 to 24 hours after release, but sperm can survive in the body for up to about 5 days. That makes the fertile window the day of ovulation plus the 5 days before it, around 6 days in total. Conception is most likely in the day or two before ovulation, when sperm are already present to meet the egg.

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: ovulation, the fertile window and trying to get pregnant.
• NHS: how long sperm survive and when conception is most likely.