Due Date by Last Period: the 280-Day Rule

Counting from the last menstrual period is the default in U.S. prenatal care: first day of your last period plus 280 days. Enter that date to get the due date, plus today’s gestational age and trimester.

The arithmetic, and the assumption underneath it

Two hundred eighty days is 40 weeks. The same result comes out of the older calendar shortcut — add 7 days, subtract 3 months, add a year — which is why this page and the Naegele’s rule calculator land in the same neighborhood.

What the number quietly assumes is spelled out in StatPearls: the technique “assumes that the patient has a normal 28-day menstrual cycle and ovulates on day 14 of that cycle.” That is one assumption stacked on another, and neither is safe. In a pooled analysis of 3,324 cycles from 581 U.S. women with no known fertility problems, ovulation was estimated to fall on cycle day 14 in only 8.7% of cycles. Mean cycle length was 30.3 days.

Worked example

Last period started January 1:

• January 1 plus 280 days lands on October 8 of the same year.
• Conception is estimated around January 15, two weeks in.
• On August 17 of that year the pregnancy is 32 weeks and 4 days along, in the third trimester.

Notice that the pregnancy is dated from a point about two weeks before conception happened. That is the convention, not an error — MedlinePlus defines gestational age as measured from the first day of the last menstrual cycle, and every prenatal milestone is set on that clock.

Why so many of these dates get moved

ACOG reports that roughly half of women recall the LMP accurately, and in the trial it cites, 40% of women who had a first-trimester ultrasound had their due date adjusted because the scan and the period disagreed by more than five days. In the group scanned only in the second trimester, just 10% were adjusted — not because those dates were better, but because a later scan is a blunter instrument.

So treat the period-based date as a working estimate that a first-trimester scan is expected to confirm or correct. The thresholds for that correction, week by week, are on due date by ultrasound.

When to use a different anchor

• You tracked ovulation with tests or temperature and know roughly when you conceived: use due date by conception date.
• The pregnancy came from IVF: the transfer date beats any period, see the IVF due date calculator.
• You are carrying twins: the 280-day arithmetic is unchanged, but the expected birth week is not — see the twin due date calculator.

Once the date is settled, the natural next question is how many days are left.

This content is informational and does not replace prenatal care. Bring your period dates and your cycle length to the first visit; both feed the date that goes in your chart.