In a 31-day cycle, ovulation usually happens around day 17, counting the first day of your period as day 1. Your fertile window runs from day 12 to day 17, and the most fertile days are day 15 and day 16.
| In a 31-day cycle | |
|---|---|
| Likely ovulation day | Day 17 (up to day 19 if your luteal phase is 12 days) |
| Fertile window | Days 12 to 17 |
| Most fertile days | Days 15 and 16 |
| Every-other-day plan | Days 12, 14, 16 and 17 |
| Expected next period | Day 32 (day 1 of the next cycle) |
How this is calculated: the luteal phase, from ovulation to your next period, is nearly fixed at 12 to 14 days. What changes from woman to woman is the first half. So you count backward: 31 days minus 14 = day 17. The fertile window is the five days before ovulation plus ovulation day.
A 31-day cycle does not ovulate on day 14. Day 14 is only right for a 28-day cycle; here it is about 3 days later.
| Cycle day | Relative to ovulation | Chance of conception |
|---|---|---|
| 12 | 5 days before | ~10% |
| 13 | 4 days before | ~16% |
| 14 | 3 days before | ~25% |
| 15 | 2 days before | ~28% |
| 16 | 1 day before | ~33% |
| 17 | Ovulation day | ~20% |
| 18 | 1 day after | <1% |
Chances by day relative to ovulation: Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995. Cycle days assume ovulation on day 17.
The most fertile day is day 16, the day before ovulation, not day 17. Sperm survives up to five days with fertile mucus; the egg lives 12 to 24 hours.
This is an estimate from your cycle length. Your body confirms it: fertile cervical mucus appears as the window opens, and an ovulation test turns positive 24 to 36 hours before ovulation. Waiting for that positive test alone means starting near the end of the window.
If your cycle length changes by more than 7 days from month to month, a calendar is not reliable: read your body directly. A variation of more than 9 days between cycles deserves a medical evaluation.
This calculation applies Chapters 13 and 14 of Map to the Positive, in Station II. The manual then teaches you to confirm the window in your own body: the five types of cervical mucus, basal temperature, ovulation tests and the six mistakes that invalidate them, and how to build your personal map.
Take 30 days. If it does not give you more clarity than everything you have read for free so far, ask for a refund and you get every cent back. We cannot promise you a pregnancy. We can promise you will not be guessing.
References: Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995.