Pregnancy Due Date Calculator

Naegele's rule, the method behind this calculator, has been a cornerstone of obstetric due-date estimation for roughly two centuries, named after the German obstetrician Franz Naegele who is credited with popularizing the specific calculation in the early 19th century, though the underlying idea of counting forward from the last menstrual period predates him. The rule itself is simple: add 280 days (40 weeks) to the first day of the last menstrual period to estimate a due date — a calculation grounded in the observation that a typical full-term pregnancy, counted from the last period rather than from actual conception, runs approximately 40 weeks.

It's worth understanding precisely why the calculation counts from the last menstrual period rather than from conception itself, since this is a common source of confusion. Actual conception typically occurs roughly two weeks after the start of the last period, around the time of ovulation in a standard cycle — but conception timing is much harder to know precisely than the more clearly identifiable start date of a last period, so obstetric convention counts pregnancy duration from the last menstrual period as a practical, consistently identifiable reference point, even though true fetal development age (sometimes called 'conceptional age') runs about two weeks behind this 'gestational age' figure. This is exactly why a due date estimate isn't simply '38 weeks from conception' in casual terms — it's 40 weeks from the last period, a distinction that matters for correctly interpreting pregnancy week milestones.

The formula's built-in assumption of a standard 28-day cycle with ovulation occurring on day 14 is the source of its main practical limitation. Real menstrual cycles vary considerably between individuals — some meaningfully shorter, some meaningfully longer than 28 days — and a cycle that deviates from this standard assumption shifts actual conception timing (and therefore the true due date) relative to what the formula, applied mechanically, would estimate. For someone with a highly regular 28-day cycle, Naegele's rule tends to be reasonably accurate; for someone with an irregular or notably different cycle length, the formula's underlying assumption doesn't hold as well, and the estimate carries correspondingly more uncertainty.

This is exactly why modern obstetric practice, while still using Naegele's rule as an initial reference point, generally relies on an early ultrasound dating scan for a more precise due date once one becomes available — ultrasound measurements of fetal size in early pregnancy correlate very reliably with gestational age, largely independent of any assumption about menstrual cycle regularity, making it a more directly physiological measurement rather than a calculation based on an assumed-standard cycle. In practice, a due date is genuinely an estimate under any method — a relatively small percentage of babies are actually born on their calculated due date, with full-term delivery understood as covering a range of roughly several weeks around that estimated date rather than a single precise day — which is worth keeping in mind regardless of which estimation method (last period, ultrasound, or a blend of both) produced the specific date being tracked.