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Pregnancy due date calculator
Estimated due date from the first day of your last period.
Quick answer
The due date is estimated by adding 280 days, or 40 weeks, to the first day of the last menstrual period. Formula: Due date = first day of the last period + 280 days (40 weeks), the Naegele rule. If it was 10 January 2026 with a 28-day cycle, the due date is Saturday, 17 October 2026.
Read the guide: How a pregnancy due date is calculated
Estimated due date
Saturday, 17 October 2026
- Likely conception
- Saturday, 24 January 2026
- Second trimester starts
- Saturday, 18 April 2026
- Third trimester starts
- Saturday, 25 July 2026
Only about 1 in 20 babies arrives on the due date. A general estimate, not medical advice. Talk to a doctor or dietitian about your own health. Your doctor will confirm the date with a scan.
How is a pregnancy due date calculated?
Doctors count pregnancy from the first day of the last period, so you are already "2 weeks pregnant" at conception.
The Naegele rule adds 280 days. If your cycle is longer or shorter than 28 days, the date shifts by the difference.
An early ultrasound can give a more exact date, and your doctor may use that instead.
What is the formula for a pregnancy due date?
- Due date = first day of the last period + 280 days (40 weeks), the Naegele rule
- For a cycle that is not 28 days, add or subtract the difference
Can you show a worked example of a pregnancy due date?
Last period began 10 January 2026, cycle 28 days: 10 January + 280 days = 17 October 2026, a Saturday.
How does cycle length change the due date?
For a last period that began on 10 January 2026. A general estimate, not medical advice.
| Cycle length | Estimated due date |
|---|---|
| 24 days | 13 October 2026 |
| 26 days | 15 October 2026 |
| 28 days | 17 October 2026 |
| 30 days | 19 October 2026 |
| 32 days | 21 October 2026 |
| 35 days | 24 October 2026 |
Common questions
How is the due date calculated?
By adding 280 days (40 weeks) to the first day of your last period.
What if my cycle is irregular?
The estimate is less reliable. A dating scan is the best way to confirm.
How accurate is a due date?
It is an estimate. Most babies arrive between 37 and 42 weeks.
When does the second trimester start?
Around week 14 and the third around week 28, counted from the last period.