Fertile Window on a 31-Day Cycle: Your Best Days
With a 31-day cycle, your fertile window is roughly cycle days 12 to 17. Those are the five days before ovulation plus ovulation day itself, the only days in the cycle when sex can lead to pregnancy.
| Cycle length | 31 days |
|---|---|
| Likely ovulation day | Cycle day 17 (range 15–19) |
| Fertile window | Cycle days 12–17 |
| Most fertile days | Cycle days 15–17 |
| Next period due | Cycle day 32 |
Why the window is six days long
Sperm can survive for up to five days, while the egg lives for about 12 to 24 hours. Pregnancy is possible when sperm are already waiting, or arrive, while the egg is viable, which creates a six-day fertile window.
Using it in practice
Because ovulation can shift by a few days from cycle to cycle, many couples have sex every one to two days starting a few days before day 12 and continuing until a day after day 17. That covers most of the variation without needing to pinpoint the exact day.
A 31-day cycle sits within the typical adult range of 21 to 35 days.
Ovulation Calculator for a 31-Day Cycle: Ovulation and fertile days for a 31-day menstrual cycle.
Open the calculatorThis article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.