Due Date on a 31-Day Cycle: How to Adjust It
Standard due date rules assume a 28-day cycle. On a 31-day cycle, ovulation happens about 3 days later than that, so your due date shifts by the same amount.
| 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 |
The formula
Due date = first day of last period + 280 days + (31 − 28) days. With a period starting on March 1, that gives a due date of December 9, 2026, and an estimated conception date of March 17.
When an ultrasound changes it
An early ultrasound measures the baby directly and is the most accurate way to date a pregnancy. If the scan disagrees with your period-based date by more than about a week in the first trimester, your provider will usually go with the scan.
A 31-day cycle sits within the typical adult range of 21 to 35 days.
Due Date Calculator: Estimate your due date from the first day of your last period.
Open the calculatorThis article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.