Three methods, three answers: why your due date is an estimate, not a deadline
Only about 4% of babies are born on their estimated due date. The rest distribute across a window of roughly two weeks on each side. The point of a “due date” is not to be right on the day — it is to anchor every other clinical decision (term vs pre-term, when to consider induction, when to monitor more closely). Worth understanding why the number you got is an estimate built on assumptions, and why three calculations can disagree.
Naegele’s rule (LMP + 280 days)
Adds 280 days to the first day of the last menstrual period. Built on two assumptions: a 28-day cycle and ovulation on day 14. If your cycle is 30 days, your true due date is about two days later. If it is 26 days, two days earlier. Most accurate when the LMP date is known with certainty (charted), not approximated as “around the 15th”.
Conception-based dating
LMP + 280 days is mathematically conception + 266 days (the assumed two weeks from period start to ovulation). If you know your conception date with certainty — IVF transfer, ovulation predictor, careful cycle tracking — drop the 14-day assumption and add 266 days directly. This is the most accurate method for IVF pregnancies, where transfer day is recorded precisely.
First-trimester ultrasound (the clinical gold standard)
Between weeks 8 and 13 of gestation, the fetus grows at a remarkably predictable rate. Measuring crown-rump length (CRL) on a first-trimester ultrasound predicts gestational age within ±5–7 days. After week 14, the accuracy of dating ultrasounds falls (biometry becomes more variable). The ACOG Committee Opinion 700 rule: if LMP-based and ultrasound-based dating differ by more than seven days in the first trimester, use the ultrasound.
When the three methods disagree
LMP says May 1, conception says May 3, ultrasound says May 8 — ACOG says use May 8. If all three agree within a few days, any of them works. The calculator above lets you compute all three on the same input so you can see the spread.
Why this matters clinically, not just emotionally
“Term” is 37–42 weeks. “Pre-term” is <37. A 38-week induction calculated on a wrong-by-two-weeks date is actually a 36-week induction — with the neonatal risks that come with prematurity. The dating decision made at the first ultrasound governs every subsequent threshold for the rest of the pregnancy. It is not paperwork.
Takeaway: First-trimester ultrasound trumps LMP-based and conception-based dating per ACOG. The calculator on this page gives all three for comparison, plus a current-week estimate and milestone timeline. If the three numbers diverge by more than seven days, your obstetric provider’s ultrasound number is the one to act on — the others are useful context, not clinical truth.
Sources: ACOG Committee Opinion 700 · NIH NICHD pregnancy information.