Due date calculator

How is the due date calculated?

By convention, the due date is 280 days — 40 weeks — after the first day of your last menstrual period, as ACOG describes. If your last period started on January 1, 2026, the due date is October 8, 2026.

What if I know the conception or IVF transfer date?

For IVF, ACOG gives 261 days after the transfer of a 5-day embryo and 263 days after a 3-day embryo. From a known conception date it is 266 days, since the 280-day rule assumes ovulation on day 14.

Can an ultrasound change my due date?

Yes. ACOG calls a first-trimester ultrasound (up to 13 weeks 6 days) the most accurate way to date a pregnancy, and changes the due date if it differs from the period-based date by more than 7 days — more than 5 days before 9 weeks.

What do early, full and late term mean?

By ACOG: early term is 37 weeks 0 days through 38 weeks 6 days, full term 39 weeks through 40 weeks 6 days, late term 41 weeks through 41 weeks 6 days, and postterm 42 weeks and beyond.

How far along am I?

Count from the first day of your last period: the calculator shows today's weeks and days and the trimester. Weeks are counted from the period, about two weeks before conception.

Why is pregnancy counted from the last period if conception is two weeks later?

Because the date of the last period is usually known and the date of conception usually is not. ACOG dates a pregnancy from the first day of the last menstrual period, so “40 weeks” includes about two weeks before conception. This is why a pregnancy that is 4 weeks along has an embryo only about two weeks old.

Is the due date the day the baby will be born?

No. It is a reference point in a range. Full term runs from 39 weeks 0 days to 40 weeks 6 days, and births before and after the due date are normal. Your due date can also change if an early ultrasound shows a different gestational age.

The arithmetic behind each starting point

From the last menstrual period the estimate is the first day of that period plus 280 days. From a known conception date it is 266 days after, because the 280-day rule assumes ovulation on day 14. For IVF it is 261 days after a 5-day embryo transfer and 263 days after a 3-day transfer. All three are on the same timeline, so the weeks and days of pregnancy are measured from the same implied period date.

Worked example: last period began February 1, 2026

Adding 280 days gives Sunday, November 8, 2026. A conception date of February 15 gives Sunday, November 8, 2026, and a 5-day embryo transferred on February 15 gives Tuesday, November 3, 2026. The 20-week point falls on Sunday, June 21, 2026, early term begins at 37 weeks on Sunday, October 18, 2026, full term at 39 weeks on Sunday, November 1, 2026 and late term at 41 weeks on Sunday, November 15, 2026. On May 10, 2026 the pregnancy would be 14 weeks and 0 days along.

Common mistakes

Entering the last day of the period rather than the first shifts the estimate by the length of the period. Assuming a 28-day cycle when yours is longer or irregular is the second: the rule counts ovulation on day 14, so with a later ovulation the real gestational age is younger than the period-based one, which is one reason a first-trimester ultrasound is used to confirm or revise the date. Third, counting in months rather than weeks gives a different answer, since months are not a fixed number of weeks.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.

Sources: American College of Obstetricians and Gynecologists, Committee Opinion No. 700 (Methods for Estimating the Due Date) and Committee Opinion No. 579 (Definition of Term Pregnancy). Related: pregnancy week calculator, conception calculator, ovulation calculator and period calculator.