How your due date is calculated

A due date is 280 days (40 weeks) after the first day of your last period. How cycle length changes it, the conception method, and why ultrasound may adjust it.

Open the Due Date Calculator

From your last period

ACOG explains that the estimated due date is 280 days (40 weeks) after the first day of the last menstrual period. This assumes a 28-day cycle. If your cycles are longer or shorter, add or subtract the difference (for a 31-day cycle, add 3 days).

Due date = first day of last period + 280 days + (cycle length − 28)

From conception

If you know the date of conception, as with IVF, the due date is about 266 days (38 weeks) after conception.

Ultrasound

ACOG recommends that an early ultrasound measurement can confirm or change the due date. Your care team will tell you which date they use.

Few babies arrive on the day

A due date is an estimate. ACOG notes that most babies are born between 38 and 41 weeks.

Frequently asked questions

How many weeks pregnant am I?

Count the weeks since the first day of your last period; the due date calculator does this for you.

Can my due date change?

Yes. An ultrasound in the first trimester can adjust it.

Sources

Last reviewed 2026-10-29. General information only, not medical, financial or legal advice.