How to work out your due date
- Pick the First day of your last period: the day bleeding started, not the day it ended.
- Change Cycle length if your cycle is usually shorter or longer than 28 days. Count from the first day of one period to the first day of the next.
- Read your estimated due date, how many weeks along you are today and the dates of a few milestones.
How the date is worked out
Take a last period that started on Sunday 1 March 2026 with a 28-day cycle. Adding 280 days gives an estimated due date of Sunday 6 December 2026. The other rows come from the same starting point:
| Milestone | Date |
|---|---|
| Conception (estimated) | Sunday 15 March 2026 |
| Second trimester begins (week 14) | Sunday 7 June 2026 |
| Full term begins (37 weeks) | Sunday 15 November 2026 |
| Due date (40 weeks) | Sunday 6 December 2026 |
With a 32-day cycle every date moves 4 days later, so the due date becomes Thursday 10 December 2026.
Naegele’s rule is often taught as a shortcut: add a year, take away three months and add seven days. Months differ in length, so the shortcut can land a day or two away from an exact 280-day count. For 1 March it gives 8 December. This calculator counts the days.
Weeks and trimesters
Pregnancy weeks are written as completed weeks plus days, so 12 weeks 4 days means you are in your 13th week. The calculator shows this and your trimester whenever today falls within the pregnancy.
The Trimester row counts the first trimester up to 13 weeks 6 days, the second up to 27 weeks 6 days and the third from 28 weeks to birth. Some sources draw the lines a week differently. Full term starts at 37 weeks, which is the date in the Full term begins row.
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