How your due date is calculated
Doctors usually estimate a due date by adding 280 days (40 weeks) to the first day of your last menstrual period. This is known as Naegele’s rule. If your cycle is longer or shorter than 28 days, the date shifts by the difference. If you know your conception date, the due date is about 266 days (38 weeks) later.
From your last period: due date = first day of last period + 280 days + (cycle length − 28 days)
From conception: due date = conception date + 266 days
How to use this calculator
- Choose how to calculate. Pick your last period or your conception date.
- Enter the date. Use the first day of your last period, not the last day.
- Set your cycle length. If you’re not sure, leave it at 28 days.
- Read your result. Your due date, current week and key dates update instantly.
Worked example
If your last period started on January 1, 2026 and your cycle is 28 days, add 280 days to get October 8, 2026. With a 32-day cycle, add 4 more days to get October 12, 2026.
Pregnancy stages by week
| Weeks | Stage |
|---|---|
| 0 – 13 weeks 6 days | First trimester |
| 14 – 27 weeks 6 days | Second trimester |
| 28 weeks to birth | Third trimester |
| 37 – 38 weeks 6 days | Early term |
| 39 – 40 weeks 6 days | Full term |
| 41 – 41 weeks 6 days | Late term |
| 42 weeks and later | Postterm |
What a due date doesn’t tell you
A due date is an estimate, not a deadline. Only around 4–5% of babies are born on their due date, and most arrive between 37 and 42 weeks. An early ultrasound is the most accurate way to date a pregnancy, so your doctor or midwife may give you a different date. Use theirs if it differs from this one.
Frequently asked questions
How is a due date calculated?
Add 280 days to the first day of your last menstrual period, adjusting for cycles longer or shorter than 28 days.
Can my due date change?
Yes. Your provider may update it after a dating ultrasound, especially if your cycles are irregular.
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