Due Date Calculator: LMP, Ultrasound and IVF

Your estimated due date is 280 days after the first day of your last period — 40 weeks. Enter that date below and the calculator returns the due date, today’s gestational age and the trimester. Then read on, because the number your clinician uses may not be this one.

Four ways a due date gets set

Obstetric practice does not have one formula. It has one anchor — 40 weeks from the last menstrual period (LMP) — and several ways to reach it, ranked by how well each one pins down the day of conception.

Starting point	The arithmetic	How tight it is
Last period	LMP + 280 days	Depends on cycle length and on recall
Early ultrasound	Measured age subtracted from the scan date, then + 280 days	About 5 to 7 days through 13 weeks 6 days
Conception or ovulation date	Conception + 266 days	As good as your estimate of ovulation
IVF transfer	Transfer + 261 days (day-5) or + 263 days (day-3)	The conception day is known

ACOG’s Committee Opinion 700 is blunt about the weak link in the first row: the 280-day convention — the modern form of Naegele’s rule — “assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day,” and it does not account for irregular cycles, variable ovulation, or hazy recall of the period date. Only about half of pregnant patients recall the LMP accurately.

Pick the row that matches what you actually know: due date by last period, due date by conception date, due date by ultrasound, or the IVF due date calculator.

When the scan replaces the period date

This is the part most calculators skip. ACOG sets a single discrepancy threshold for each window of pregnancy: below it the LMP date stands, above it the ultrasound date wins.

Gestational age at the scan	Ultrasound overrides the LMP if the gap is
Before 9 weeks 0 days	more than 5 days
9 weeks 0 days to 13 weeks 6 days	more than 7 days
14 weeks 0 days to 15 weeks 6 days	more than 7 days
16 weeks 0 days to 21 weeks 6 days	more than 10 days
22 weeks 0 days to 27 weeks 6 days	more than 14 days
28 weeks 0 days and beyond	more than 21 days

Two consequences follow. First, an early scan moves the date far more often than a late one — in the trial ACOG cites, 40% of women scanned in the first trimester had the due date adjusted, against 10% of those scanned in the second. Second, a pregnancy that reaches 22 weeks with no scan confirming or revising the date is formally “suboptimally dated,” which changes how the rest of the pregnancy is managed.

What the due date actually predicts

It predicts week 40, not a birthday. The 2023 U.S. birth file shows where deliveries really land:

Gestational age at birth	Share of U.S. births, 2023
Preterm, under 37 weeks	10.41%
Early term, 37 to 38 weeks	29.84%
Full term, 39 to 40 weeks	54.94%
Late term, 41 weeks	4.56%
Post-term, 42 weeks and over	0.26%

Those labels are not decoration. ACOG and SMFM replaced the single word “term” with these more specific ones, and NICHD built a national campaign around the change: full term now starts at 39 weeks, because babies do best when born during weeks 39 and 40. Read the due date as the midpoint of the biggest row in that table, not as an appointment.

Once you have a date, the follow-up questions are usually how many days are left and how the weeks map onto months.

This content is informational and does not replace prenatal care. The due date of record for your pregnancy is the one your clinician assigns and documents.