Pregnancy Due Date Calculator

Based on Naegele's rule

This estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period — the standard rule of thumb used as a starting estimate, first described by German obstetrician Franz Naegele in the 1800s.

It's a general estimate, not a substitute for dates confirmed by an ultrasound or your healthcare provider — actual delivery timing varies for plenty of normal reasons, and only about 1 in 20 births actually lands on the exact estimated date.

How the trimesters break down

The first trimester runs from week 1 through the end of week 13, the second from week 14 through week 27, and the third from week 28 until delivery, which is considered full-term anywhere from week 37 to week 42. The "current week" figure above gives a rough sense of where in that range you are, based purely on the last-period date entered.

Why cycle length changes the real answer

Naegele's rule assumes a textbook-average 28-day cycle with ovulation on day 14 — anyone with a longer or shorter cycle will conceive on a different day relative to their last period, which shifts the true due date away from this estimate in the same direction as their cycle length differs from 28 days. Someone with a consistently 35-day cycle, for example, tends to ovulate later and so tends to have a due date later than this rule predicts. This is exactly why an early ultrasound (which measures the fetus directly rather than working backward from a remembered date) is generally considered more accurate than a last-period-based estimate, especially for anyone with irregular cycles.

An estimate to plan around, not a deadline

Even with a precisely known conception date, only a small fraction of births land on the exact estimated day — full-term is a five-week window (weeks 37 through 42), not a single date, and first pregnancies in particular skew slightly later than the estimate on average. Use the number here for planning purposes (booking leave, preparing a nursery, scheduling checkups) rather than as a fixed appointment to expect delivery on.