Due Date on a 21-Day Cycle: How to Adjust It
Standard due date rules assume a 28-day cycle. On a 21-day cycle, ovulation happens about 7 days earlier than that, so your due date shifts by the same amount.
| Cycle length | 21 days |
|---|---|
| Likely ovulation day | Cycle day 7 (range 5–9) |
| Fertile window | Cycle days 2–7 |
| Most fertile days | Cycle days 5–7 |
| Next period due | Cycle day 22 |
The formula
Due date = first day of last period + 280 days + (21 − 28) days. With a period starting on March 1, that gives a due date of November 29, 2026, and an estimated conception date of March 7.
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 21-day cycle is at the short end of the typical adult range of 21 to 35 days. Short cycles can mean ovulation happens very soon after your period ends, so the fertile window may overlap the last days of bleeding. If your cycles are regularly shorter than 21 days, mention it to a doctor.
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.