Pregnancy Due Date

Estimates a baby's due date from last menstrual period, conception date, or ultrasound gestational age.

The due date is an estimate — a 40-week target, not a deadline. Most births occur between 38 and 42 weeks.

1. Select calculation method and enter the relevant date(s).
2. For ultrasound, add gestational age in weeks and days.
3. Review estimated due date and current gestational age.

Naegele's rule

Due date = estimated first day of the last period + 280 days. From a conception date, that first day is 14 days earlier. From an ultrasound, that first day is the ultrasound date counted back by the weeks and days entered.

Source. The due date is 280 days after the estimated first day of the last menstrual period, which is 40 weeks. A conception date is treated as 14 days after that first day. An ultrasound date is counted back by the weeks and days entered, and that day is the estimated first day. The American College of Obstetricians and Gynecologists describes pregnancy as lasting 40 weeks. ACOG, how a fetus grows.

Updated November 1, 2026

Last period starting September 1 → estimated due date around June 8.

• Only about 5% of babies arrive on the exact due date.
• Always confirm with your healthcare provider.

How accurate is a due date from my last period?

Naegele's rule assumes a regular 28-day cycle. Irregular cycles or uncertain LMP dates reduce accuracy — first-trimester ultrasound is often more reliable.

Can my due date change after an ultrasound?

Yes. Early ultrasound dating can shift the due date if it differs from LMP-based estimates. Your provider will explain which date to use.