Pregnancy Due Date Calculator

Pregnancy due dates are typically estimated from the first day of the last menstrual period (LMP), using a standard 40-week (280-day) pregnancy length. This calculator applies that standard method, known as Naegele's rule, to give a quick estimate.

The formula (Naegele's rule)

This assumes a typical 28-day menstrual cycle with ovulation around day 14. For people with different cycle lengths, the actual conception date — and therefore the true due date — may fall earlier or later than this estimate suggests.

How accurate is this estimate?

Only about 5% of babies are born on their exact calculated due date. Most full-term births occur within a window of roughly two weeks before to two weeks after the estimated date. This is a normal, expected range, not a sign that anything is off — due dates are a statistical estimate, not a precise prediction.

Other ways due dates are estimated

• Last menstrual period (LMP): the method used here — simple but assumes a standard cycle length.
• Early ultrasound: generally considered more accurate than LMP alone, particularly for people with irregular cycles, since it measures the embryo or fetus directly.
• IVF transfer date: for pregnancies conceived via IVF, the due date can be calculated precisely from the known embryo transfer date.

See our full guide for more on how this calculation works and its assumptions.

Common mistakes

• Treating the due date as exact. It's a statistical estimate — actual delivery commonly occurs anywhere within a two-week window on either side.
• Ignoring irregular cycles. Naegele's rule assumes a 28-day cycle; people with longer or shorter cycles may have a due date that shifts earlier or later than this simple calculation suggests.

References

• American College of Obstetricians and Gynecologists (ACOG) — clinical guidance on pregnancy dating

Frequently asked questions