If you are trying to conceive, knowing roughly when you ovulate helps. But the fertile window is widely misunderstood — and understanding it properly takes some pressure off.

→ Use our fertile period calculator

The fertile window is not one day

An egg survives about 24 hours after it is released. If that were the whole story, the fertile window would be a single day.

But sperm survive far longer — up to about 5 days in the female reproductive tract. So sperm that arrive several days before ovulation can still be waiting when the egg is released.

This means your fertile window is roughly six days: the five days before ovulation, plus the day of ovulation itself. Sex in the two or three days before ovulation is at least as likely to result in pregnancy as sex on the day.

Working out when you ovulate

Ovulation happens about 14 days before your next period — not on day 14 of your cycle. Those are only the same thing if your cycle is exactly 28 days.

  • 28-day cycle → ovulation around day 14, fertile window roughly days 9–14
  • 32-day cycle → ovulation around day 18, fertile window roughly days 13–18
  • 24-day cycle → ovulation around day 10, fertile window roughly days 5–10

Counting forward from your period misleads you if your cycle is not 28 days. Counting back from the next one is more reliable.

Signs you can watch for

  • Cervical mucus becomes clear, wet and slippery around your fertile days. This is the most practical everyday sign and costs nothing.
  • Ovulation predictor kits detect the hormone surge in urine and give a day or so of warning.
  • Basal body temperature rises slightly after ovulation — useful to confirm it happened, but too late to act on.
  • Mid-cycle twinge on one side, in some women.

You may not need to time anything

This is worth saying clearly, because tracking can take over.

Having sex every two to three days throughout your cycle is as effective as carefully timing to your fertile window — and it means there is always sperm present when ovulation happens, whether or not your prediction was right. For most couples this is the better approach: it works, and it does not turn your relationship into a schedule.

Calculators and kits are most useful if your cycles are long, if you are apart for parts of the month, or if you simply prefer knowing.

If your cycles are irregular

Any calculator assumes a reasonably regular cycle. If yours varies a lot, the estimate will not be reliable — and irregular cycles can themselves signal that ovulation is not happening consistently. That is worth investigating rather than working around. See our guide to anovulation.

When to see a doctor

See a doctor after a year of trying, or six months if you are over 35. Go sooner if your periods are irregular or absent, or if you have a known gynaecological condition — there is no benefit in another six months of careful timing if the underlying issue is that ovulation is not happening.

The bottom line

Your fertile window is about six days, ending on the day you ovulate — and ovulation is roughly 14 days before your next period, not day 14. Sex every two to three days across the cycle works as well as precision timing. Use the calculator as a guide, not a rule.

→ Calculate your fertile period

References

  1. National Health Service. Fertility in the menstrual cycle [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
  2. National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NG257. London: NICE; 2026. Available from: https://www.nice.org.uk/guidance/ng257