In a 27-day cycle, ovulation usually happens around day 13, counting the first day of your period as day 1. Your fertile window runs from day 8 to day 13, and the most fertile days are day 11 and day 12.
| In a 27-day cycle | |
|---|---|
| Likely ovulation day | Day 13 (up to day 15 if your luteal phase is 12 days) |
| Fertile window | Days 8 to 13 |
| Most fertile days | Days 11 and 12 |
| Every-other-day plan | Days 8, 10, 12 and 13 |
| Expected next period | Day 28 (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: 27 days minus 14 = day 13. The fertile window is the five days before ovulation plus ovulation day.
A 27-day cycle does not ovulate on day 14. Day 14 is only right for a 28-day cycle; here it is about 1 days earlier.
| Cycle day | Relative to ovulation | Chance of conception |
|---|---|---|
| 8 | 5 days before | ~10% |
| 9 | 4 days before | ~16% |
| 10 | 3 days before | ~25% |
| 11 | 2 days before | ~28% |
| 12 | 1 day before | ~33% |
| 13 | Ovulation day | ~20% |
| 14 | 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 13.
The most fertile day is day 12, the day before ovulation, not day 13. 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.