Pregnancy Due Date Calculator

This calculator estimates a pregnancy due date using Naegele's rule, a standard method that adds 280 days (40 weeks) to the first day of your last menstrual period (LMP) — the same method most healthcare providers use for an initial estimate before an ultrasound is available.

The 280-day figure comes from a typical pregnancy lasting about 38 weeks from conception, plus roughly 2 extra weeks accounting for the fact that pregnancy is conventionally dated from the LMP rather than the (harder to pin down precisely) conception date itself, which for a standard 28-day cycle occurs around 2 weeks after the period starts.

This estimate assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. For people with longer, shorter, or irregular cycles, the actual conception date — and therefore the true due date — can shift meaningfully from what LMP-based Naegele's rule predicts, which is one reason this is treated as a starting estimate rather than a precise prediction.

In practice, only around 5% of babies are born exactly on their calculated due date — a full-term pregnancy is considered normal anywhere from 37 to 42 weeks, meaning the "due date" is really the center of a several-week range rather than a specific day to expect.

Once a pregnancy is confirmed, an early ultrasound (typically in the first trimester) is generally considered more accurate than LMP-based dating alone, especially for anyone with irregular cycles, since it measures the fetus directly rather than relying on an assumed cycle length — this is why due dates sometimes get adjusted after the first ultrasound.

The 40-week estimate breaks into three trimesters: roughly weeks 1–13 (first trimester), 14–27 (second trimester), and 28–40 (third trimester) — a due date calculator like this one is often used alongside that breakdown to track overall pregnancy progress.