Pregnancy Due Date Calculator

Clinically, the length of a pregnancy is measured in "gestational age," counted from the first day of the last menstrual period — not from the estimated moment of conception, which typically follows about two weeks later. A full-term pregnancy is conventionally set at 280 days, or 40 weeks, from that starting point. The convention exists mostly for practical reasons: the first day of a period is a specific, rememberable date for most people, while the exact moment of conception essentially never is.

The best-known due-date formula, known as Naegele’s rule after the 19th-century obstetrician credited with popularizing it, simply adds 280 days to the first day of the last period. It assumes a textbook 28-day cycle with ovulation occurring around day 14, so a longer or shorter average cycle shifts the estimated ovulation date — and with it, the due date — by roughly the number of days the cycle differs from 28.

When a conception date, an early ultrasound measurement, or an IVF embryo transfer date is available instead, the estimate is built from that fixed point rather than from an assumption about ovulation timing. A first-trimester ultrasound measures the size of the developing embryo or fetus against well-established growth curves, which is generally considered the most precise of the common dating methods since it does not depend on cycle regularity or memory of the last period at all.

Pregnancy is traditionally split into three trimesters, each spanning roughly thirteen weeks. The divisions are a shorthand for describing when certain developmental milestones and common physical changes tend to cluster, not precise biological cutoffs — nothing distinct happens at exactly week 13 or week 27 for every pregnancy.

Every dating method here produces a single calculated date, but actual delivery timing spreads out around it: only a small minority of pregnancies deliver on the exact estimated date, and a full-term delivery anywhere from about three weeks before to two weeks after it is considered entirely normal. The estimate is best read as the midpoint of a wide, well-studied range rather than a prediction of a specific day — which is also why the precision of the underlying method (last period versus an early ultrasound, for example) matters more than any extra decimal place a calculation can produce.