Pregnancy can only happen in a short window each cycle – the days just before ovulation and the day of ovulation itself. How many days after your period that window opens depends mainly on the length of your cycle. This guide explains how to estimate it.
Important
counting days is not a reliable method of contraception, and this article is general information, not medical advice. Talk to a doctor, nurse or sexual health clinic about contraception or trying to conceive.
How the fertile window works
- Ovulation is when an ovary releases an egg. The NHS says it usually happens 10 to 16 days before your next period starts.
- The egg survives for about a day after ovulation.
- Sperm can survive inside the body for several days – the NHS says up to 7 days.
So the fertile window is roughly the five or six days before ovulation plus the day of ovulation. Sex a few days before you ovulate can lead to pregnancy because sperm are still there when the egg is released.
Estimating your ovulation day
Estimated ovulation day ≈ Cycle length − 14 (day 1 = first day of your period)
In a 28-day cycle, ovulation is around day 14, so the most fertile days are about days 9 to 14. Because ovulation can happen anywhere from 10 to 16 days before the next period, real dates vary.
| Cycle length | Estimated ovulation (day) | Estimated fertile window (days) | Days after a 5-day period |
|---|---|---|---|
| 21 days | 7 | 2–7 | Fertile days can overlap with the period |
| 24 days | 10 | 5–10 | From the last day of bleeding |
| 28 days | 14 | 9–14 | About 4 days after the period ends |
| 32 days | 18 | 13–18 | About 8 days after |
| 35 days | 21 | 16–21 | About 11 days after |
These are averages. Ovulation shifts with stress, illness, travel and other factors, and many people’s cycles vary by several days from month to month.
Try it with your numbersPeriod CalculatorPredict your next periods, likely ovulation day and fertile window from your last period and cycle length, and learn…Open calculatorCan you get pregnant right after your period?
It is possible but less likely. With a short cycle, ovulation can happen only a few days after bleeding stops, and sperm from sex near the end of a period may survive until then. The NHS notes that getting pregnant soon after a period is possible, especially for people with short cycles.
Signs that ovulation is near
- Cervical mucus becomes clear, wet and stretchy, like raw egg white.
- Ovulation predictor kits detect the rise in luteinising hormone (LH) a day or two before ovulation.
- Basal body temperature rises slightly after ovulation – useful for confirming it has happened, not for predicting it.
- Some people feel a twinge of pain on one side of the lower abdomen.
Irregular cycles
If your cycle length changes a lot, counting days is much less accurate. Tracking several cycles, using ovulation tests and watching cervical mucus give a better picture. See a doctor if your periods are very irregular, stop for three months or more, or if you have been trying to conceive for a year (or six months if you are 35 or older).
Track your own cycle
Enter the first day of your last period and your average cycle length into the period calculator to see your estimated next periods and fertile windows. If you think you might be pregnant, read when to take a pregnancy test.
Frequently asked questions
In a 28-day cycle, the fertile window is about days 9–14, roughly four days after a five-day period ends. With shorter cycles it can start earlier.
It is unlikely but possible, especially with a short cycle, because sperm can survive for several days and ovulation may come early.
The two or three days before ovulation and the day of ovulation. Estimate ovulation as cycle length minus 14.
No. Ovulation varies from cycle to cycle, so counting days alone is not a reliable way to prevent pregnancy.
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