How ovulation and the fertile window are worked out
Ovulation usually happens about 14 days before the next period starts, so the calculator subtracts the luteal phase (the days from ovulation to your period) from your cycle length. From that date, the fertile window is counted as six days ending on ovulation day: the five days before it and the day itself. Sperm can survive in the body for days (womenshealth.gov says up to three, other sources up to five), while the egg lives only a short time after release, and the chance of pregnancy is highest before ovulation and in the 12 to 24 hours after it.
Example: the last period began on 2 March 2026 and the cycle is 28 days. Ovulation is 2 March + 28 − 14 = 16 March, the fertile window runs from 11 to 16 March, and the next period is due on 30 March. With a 32-day cycle, ovulation moves to 20 March, the window to 15–20 March and the next period to 3 April. Ovulation is not always on day 14; it follows your cycle length.
How to read the result
- The best days are the last two or three of the window, just before and on ovulation day.
- Ovulation can shift from month to month in the same woman; womenshealth.gov says the days before ovulation can range from 13 to 20. Treat the dates as an estimate.
- The next three cycles are shown for early planning, and they get less reliable the further ahead you look.
Common mistakes
- Using it as contraception. It is for planning a pregnancy only, and pregnancy can happen outside the days shown.
- Assuming ovulation is always on day 14.
- Relying on the calculator with irregular cycles. Other signs, such as cervical mucus and body temperature, help then, and womenshealth.gov suggests subtracting 18 from your shortest cycle and 11 from your longest to get a wider window.
When to see a doctor
womenshealth.gov advises seeing a doctor if you are under 35 and have not conceived after a year of regular sex without birth control, or 35 or older and not after six months, or earlier if you suspect a fertility problem. Also see one if your cycles are shorter than 21 days, longer than 35, or irregular.
This calculator gives general information, not a diagnosis or medical advice.
Sources
- womenshealth.gov: trying to conceive — cycle length, ovulation timing, how long sperm live, when to see a doctor.
- NHS: fertility in the menstrual cycle — ovulation usually 10 to 16 days before the next period; sperm can survive up to 7 days.
Planning a pregnancy is easier with early medical follow-up. In MyClinic (Sehaty) you search for a doctor by speciality and location and book without a phone call, and your medical file holds your history.
How to book a doctor online in Egypt · See MyClinic · Request a demo
Questions
How do I calculate my ovulation day?
Subtract 14 days from the date your next period is due. In a 28-day cycle that is 14 days after the first day of your period, and in a 32-day cycle it is 18 days after.
What is the fertile window?
About six days ending on ovulation day, because sperm survive in the body for days and the egg lives only briefly. The chance is highest before ovulation and in the 12 to 24 hours after.
Can I use this as contraception?
No. It is for planning a pregnancy only, and the estimate can be wrong because ovulation timing varies. Ask your doctor about contraception.
What if my periods are irregular?
The date is harder to predict and can change from cycle to cycle. Other signs such as cervical mucus or body temperature can help, and see a doctor if your cycles are shorter than 21 days, longer than 35 or irregular.
How many days are there between ovulation and the period?
Usually 14 to 16 days according to womenshealth.gov. The calculator uses 14 unless you enter your own.
When should I see a doctor if I am not getting pregnant?
After a year of regular sex without birth control if you are under 35, after six months if you are 35 or older, or sooner if you have a concern.
Read the guide
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