How is a due date calculated?

Bottom lineA due date estimates delivery at about 40 weeks (280 days) from the first day of your last period, assuming a 28-day cycle with day-14 ovulation; it can be adjusted for longer or shorter cycles or a known conception date, and a first-trimester dating ultrasound is most accurate. It's only an estimate, since normal delivery ranges from 37 to 42 weeks and few babies arrive on the exact date.

A due date is an estimate of when your baby is likely to arrive, about 40 weeks from the first day of your last period. Only about 1 in 20 babies actually arrives on the exact date.

The standard method (Naegele's rule)

• Counted as 40 weeks (280 days) from the first day of your last menstrual period (LMP)
• This assumes a regular 28-day cycle with ovulation around day 14
• Pregnancy is dated from LMP by convention, even though conception happens about 2 weeks later

Adjusting for your cycle

If your cycle is longer or shorter than 28 days, or you know your conception or ovulation date, the estimate can be adjusted, since ovulation timing shifts the true date.

The most accurate method

A dating ultrasound in the first trimester (around 8 to 13 weeks) is the most accurate way to set or confirm a due date, especially with irregular cycles.

Why it's an estimate

A normal pregnancy can deliver anywhere from 37 to 42 weeks. The due date is a target, not a deadline.

What to do

Use the Due Date Calculator for an LMP-based estimate, or the Conception Date Calculator if you know when you conceived. A clinician confirms it with an ultrasound.

Femora tracks your last period and cycle length, the inputs a due-date estimate is built on.