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.

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