Pregnancy due date calculator
Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, a method called Naegele's rule. If your last period began January 1, 2026, your due date is October 8, 2026. If you know your conception date, add 266 days instead. Enter either date below to get yours.
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Estimate only. About 5% of babies arrive on the due date, and an ultrasound in the first trimester is the most accurate way to date a pregnancy. This tool is not a substitute for medical advice.
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How the due date is calculated
The calculator uses Naegele's rule: take the first day of your last menstrual period (LMP), add one year, subtract three months, and add seven days. That equals 280 days, or 40 weeks. It assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the result shifts by the difference.
Due date = LMP + 280 days + (cycle length − 28) Due date = conception date + 266 days Gestational age (weeks) = days since LMP ÷ 7
Doctors count gestational age from the first day of the last period, so you are already "2 weeks pregnant" at conception. A pregnancy at 12 weeks is about 10 weeks after fertilization.
Trimesters and full term
| Stage | Gestational age |
|---|---|
| First trimester | through 13 weeks 6 days |
| Second trimester | 14 weeks to 27 weeks 6 days |
| Third trimester | 28 weeks until birth |
| Early term | 37 weeks to 38 weeks 6 days |
| Full term | 39 weeks to 40 weeks 6 days |
| Preterm | before 37 weeks |
Appointment and ultrasound timing varies by provider. The windows shown are typical, not a schedule.
Why the due date is rarely the birth date
Only about 5% of babies are born on their due date; most arrive within two weeks either side. First pregnancies often run a little longer. If the first-trimester ultrasound differs from your LMP date by more than about 7 days, your provider will usually use the ultrasound date instead.
3 real-life examples
Last period began January 1, 2026, 28-day cycle
January 1 + 280 days = October 8, 2026 (Naegele: January 1 + 1 year − 3 months = October 1, plus 7 days). On March 5 you would be 9 weeks 0 days pregnant.
Last period began March 15, 2026, 28-day cycle
March 15 + 280 days = December 20, 2026. Early term starts November 29 (37 weeks), full term starts December 13 (39 weeks), and 41 weeks 6 days ends January 2, 2027.
Conception on January 15, 2026
January 15 + 266 days = October 8, 2026. That matches a last period on January 1, with conception 14 days into the cycle.