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.
How the due date is calculated
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.
Common questions
It marks the single most likely day, but only about 4–5% of babies are born exactly on it. Roughly 9 in 10 arrive within two weeks either side. The estimate is most reliable when your cycles are regular and it’s confirmed by an early ultrasound.
Yes, and it’s common. If a first-trimester ultrasound shows the baby measuring more than about a week off the period-based date, your doctor will usually update the EDD to match the scan. After 14 weeks ultrasound dating gets less precise, so early changes tend to be final.
An early ultrasound (before 14 weeks) beats counting from the last period, because it measures the baby directly instead of assuming a 28-day cycle. Guidelines recommend using the ultrasound date when it clearly disagrees with the period-based one. Your doctor weighs both.
Naegele’s rule assumes ovulation on day 14, so a longer cycle pushes ovulation — and your true due date — later, and a shorter one earlier. This calculator already adds or subtracts the difference for you. An ultrasound removes the uncertainty entirely.
No — from the last period. Gestational age starts on the first day of your LMP, about two weeks before conception actually occurs. So at the moment of conception you’re already “two weeks pregnant”, and a baby born at 40 weeks is really about 38 weeks old.
Full term is 39 weeks to 40 weeks 6 days, when outcomes are best. The broader healthy range is 37–42 weeks: 37–38 is “early term”, week 41 is “late term”, and past 42 is “post-term”. Birth before 37 weeks is considered preterm.
ℹ️Medical disclaimer. This tool is for general information only and isn’t medical advice, diagnosis or treatment. Estimates can differ from your real dates — always confirm them with your doctor or midwife.