Due Date on a 28-Day Cycle: How to Adjust It
A 28-day cycle is exactly what standard due date rules assume, so no adjustment is needed: add 280 days to the first day of your last period.
| Cycle length | 28 days |
|---|---|
| Likely ovulation day | Cycle day 14 (range 12–16) |
| Fertile window | Cycle days 9–14 |
| Most fertile days | Cycle days 12–14 |
| Next period due | Cycle day 29 |
The formula
Due date = first day of last period + 280 days + (28 − 28) days. With a period starting on March 1, that gives a due date of December 6, 2026, and an estimated conception date of March 14.
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 28-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.