Ovulation in a 37-day cycle: day 23
With a 37-day cycle, ovulation would be around day 23 if it happens regularly.
Regularly long cycles are sometimes linked to polycystic ovary syndrome (PCOS), so if you also have acne, excess hair growth or difficulty conceiving, a doctor can check.
Ovulation is usually about 14 days before the next period, so in a 37-day cycle it falls on day 23 — 9 days later than in a 28-day cycle. A 37-day cycle is longer than the usual adult range of about 21–35 days. Longer cycles usually mean ovulation happens later — and sometimes less regularly — so it is worth talking to a doctor or nurse if your cycles are regularly this long, especially if you are trying to conceive.
Your 37-day cycle, phase by phase
| Phase | Cycle days |
|---|---|
| Period (typical) | Days 1–5 |
| Follicular phase | Days 1–22 (22 days) |
| Fertile window | Days 18–24, best on days 21–23 |
| Ovulation | Day 23 |
| Luteal phase | Days 24–37 (14 days) |
Start LH ovulation tests around day 20; a positive result on day 21 or 22 points to ovulation on day 23.
If your cycle is sometimes 35 or 39 days
Few cycles are identical every month. If yours ranges between 35 and 39 days, ovulation could fall anywhere from day 21 to day 25, which widens your possible fertile days to days 16 to 26. Base the start of ovulation testing on your shortest recent cycle (day 18), and expect your period between day 36 and day 40.
| Cycle length | Ovulation | Fertile window |
|---|---|---|
| 35 days | Day 21 | Days 16–22 |
| 36 days | Day 22 | Days 17–23 |
| 37 days | Day 23 | Days 18–24 |
| 38 days | Day 24 | Days 19–25 |
| 39 days | Day 25 | Days 20–26 |
Example: a 37-day cycle starting today
| Period starts (day 1) | |
|---|---|
| Start ovulation tests (day 20) | |
| Fertile window (days 18–24) | – |
| Estimated ovulation (day 23) | |
| Next period / test day (day 38) |
Test for pregnancy from day 38 — see the Pregnancy Test Calculator for exact dates.