A last period that began on 2026-01-01 gives an estimated due date of 2026-10-08, which is 280 days or 40 weeks later.
From the last period
The standard method adds 280 days to the first day of the last menstrual period. That counts 40 weeks from that date, even though conception happens about two weeks later.
Starting on 2026-01-01, the estimated due date is 2026-10-08.
From a known conception date
If the conception date is known, the calculator adds 266 days instead. Conception on 2026-01-15 gives 2026-10-08, the same date as the example above, because the two methods differ by 14 days.
An estimate, not a deadline
Only a small share of babies arrive on the estimated date. Most arrive in the weeks around it. Your care provider may adjust the date after an ultrasound, so rely on their guidance for follow-up.
| Last period began | Estimated due date |
|---|---|
| 2026-01-01 | 2026-10-08 |
| 2026-03-15 | 2026-12-20 |
| 2026-06-01 | 2027-03-08 |
| 2026-09-10 | 2027-06-17 |
FAQ
How many weeks is a pregnancy?+
About 40 weeks counted from the first day of the last menstrual period.
Why count from the last period and not conception?+
The period date is usually easier to remember than the date of conception, so it is the usual starting point.
Can the due date change?+
Yes. A care provider can revise it, for example after an early ultrasound.