Your estimated due date (EDD) is the single most likely birthday: 40 weeks from the first day of your last period, or 38 weeks from conception. Enter either date below to see it — plus your current week and trimester. Remember it’s an estimate, not an appointment: only about 1 in 20 babies arrives exactly on the day, and an early ultrasound is the most precise way to confirm it.
Most due dates come from Naegele’s rule — the standard obstetricians have used for over a century: take the first day of your last menstrual period and add 280 days, i.e. 40 weeks. If you know the conception day, the due date lands 266 days later — conception typically happens about two weeks after a period starts, which is why the two methods differ by roughly two weeks. Both assume a “textbook” 28-day cycle with ovulation around day 14, so this calculator also shifts the estimate by however much your cycle differs from 28 days.
The most precise dating method of all is an early ultrasound. Before 14 weeks, measuring the embryo’s crown–rump length pins gestational age to within a few days, so doctors often update an LMP-based date after the first scan. However it’s computed, the EDD marks the middle of a healthy 37–42-week window — not a fixed deadline. Always confirm your dates with your doctor.