Ovulation in a 31-day cycle: day 17
With a 31-day cycle, ovulation is around day 17 — three days after the textbook day 14 — and your period is due on day 32.
Basal body temperature charting is useful at this length: the rise after ovulation shows whether you really ovulate around day 17 or a little earlier or later.
Ovulation is usually about 14 days before the next period, so in a 31-day cycle it falls on day 17 — 3 days later than in a 28-day cycle. A 31-day cycle is within the usual adult range of about 21–35 days.
Your 31-day cycle, phase by phase
| Phase | Cycle days |
|---|---|
| Period (typical) | Days 1–5 |
| Follicular phase | Days 1–16 (16 days) |
| Fertile window | Days 12–18, best on days 15–17 |
| Ovulation | Day 17 |
| Luteal phase | Days 18–31 (14 days) |
Start LH ovulation tests around day 14; a positive result on day 15 or 16 points to ovulation on day 17.
If your cycle is sometimes 29 or 33 days
Few cycles are identical every month. If yours ranges between 29 and 33 days, ovulation could fall anywhere from day 15 to day 19, which widens your possible fertile days to days 10 to 20. Base the start of ovulation testing on your shortest recent cycle (day 12), and expect your period between day 30 and day 34.
| Cycle length | Ovulation | Fertile window |
|---|---|---|
| 29 days | Day 15 | Days 10–16 |
| 30 days | Day 16 | Days 11–17 |
| 31 days | Day 17 | Days 12–18 |
| 32 days | Day 18 | Days 13–19 |
| 33 days | Day 19 | Days 14–20 |
Example: a 31-day cycle starting today
| Period starts (day 1) | |
|---|---|
| Start ovulation tests (day 14) | |
| Fertile window (days 12–18) | – |
| Estimated ovulation (day 17) | |
| Next period / test day (day 32) |
Test for pregnancy from day 32 — see the Pregnancy Test Calculator for exact dates.