How the due date is calculated (Naegele's rule)
The estimated due date is usually found by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is known as Naegele's rule. If you know the conception date, add 266 days instead.
| Based on | Formula | Example |
|---|---|---|
| Last period (LMP) | LMP + 280 days | Jan 1, 2026 → Oct 8, 2026 |
| Conception date | Conception + 266 days | Jan 15, 2026 → Oct 8, 2026 |
| Cycle adjustment (optional) | LMP + 280 + (cycle − 28) | A 32-day cycle moves it 4 days later |
If your average cycle is not 28 days, enter it and the due date shifts by (cycle − 28) days.
Weeks pregnant and trimesters
Weeks pregnant are counted from the LMP to today and shown as "N weeks M days". If you enter a conception date, the same reference point (due date − 280 days) is used, so the week count stays consistent.
- 1st trimester: week 0 to 13 weeks 6 days
- 2nd trimester: week 14 to 27 weeks 6 days
- 3rd trimester: week 28 to birth
The results show your due date and weekday, current weeks pregnant, trimester, a countdown, and a table of the dates you reach weeks 12, 14, 20, 28, 37 (full term begins) and 40. Past milestones are dimmed. To count days to any other date, try the date calculator.
Things to keep in mind
- A due date is an estimate; giving birth some days before or after it is common.
- Your clinic may revise the due date based on an ultrasound. If so, go with the date your care provider gives you.
- This tool is for general information and does not replace medical advice. Everything is calculated in your browser and nothing is stored.