How due dates are estimated — and why they're estimates
A pregnancy due date is calculated, not measured — and despite the precision of a single calendar date, it's genuinely an estimate with a fairly wide normal range around it. This calculator uses Naegele's rule, the standard method taught in obstetrics: add 280 days (40 weeks) to the first day of your last menstrual period.
The logic behind 280 days: a typical pregnancy lasts about 38 weeks from conception, but because ovulation usually happens around day 14 of a 28-day cycle, conception itself is about two weeks after the last period started — so counting from the last period (which is easier for most people to know precisely) adds two extra weeks, landing on 40 weeks total.
Why so few babies arrive exactly on time
Only a small percentage of babies are born precisely on their calculated due date — most arrive within a window of about two weeks before or after. This isn't a sign anything is wrong; full-term birth is generally considered anywhere from 37 to 42 weeks, a five-week range that the single due date doesn't communicate on its own.
When ultrasound dating differs
Early ultrasound measurements (typically in the first trimester) measure the embryo or fetus directly and are generally considered more accurate than last-period-based calculations, particularly for people with irregular cycles. If your healthcare provider's ultrasound-based due date differs from this calculator's estimate by a few days, that's normal and the ultrasound date is usually the one used going forward.