Pregnancy Due Date Calculator
Enter the first day of your last period. An estimate only, not medical advice.
How the estimate works
Naegele’s rule adds 280 days, or 40 weeks, to the first day of the last menstrual period. Pregnancy weeks are counted from that date, about two weeks before conception usually happens, which is why a pregnancy is said to last 40 weeks rather than 38. For a last period starting January 1, 2026, the estimated due date is October 8, 2026.
Key dates for that example
| Milestone | Gestational age | Date (last period January 1, 2026) |
|---|---|---|
| End of first trimester | 13 weeks + 6 days | April 8, 2026 |
| Start of third trimester | 28 weeks | July 16, 2026 |
| Early term begins | 37 weeks | September 17, 2026 |
| Full term begins | 39 weeks | October 1, 2026 |
| Estimated due date | 40 weeks | October 8, 2026 |
| Late term begins | 41 weeks | October 15, 2026 |
| Post-term begins | 42 weeks | October 22, 2026 |
Term definitions
The American College of Obstetricians and Gynecologists defines early term as 37 weeks to 38 weeks 6 days, full term as 39 weeks to 40 weeks 6 days, late term as 41 weeks to 41 weeks 6 days, and post-term as 42 weeks or later. Only a small share of babies arrive on the estimated date itself; most are born within the two weeks either side of it.
Why your provider’s date may differ
The rule assumes a 28-day cycle with ovulation around day 14. With a longer or shorter cycle, ovulation and the real due date shift by about the same number of days. A dating ultrasound in the first trimester measures the baby directly and is generally more accurate, so if your provider gives a different date, use theirs.
Sources: Naegele’s rule; ACOG Committee Opinion No. 579, Definition of Term Pregnancy. This calculator gives an estimate for planning only and is not medical advice.