Pregnancy Due Date Calculator — FAQ

By default it adds 280 days (40 weeks) to the first day of your last menstrual period — the standard Naegele's rule used clinically worldwide. You can also use a conception or ovulation date (adds 266 days) or an ultrasound date with the gestational age your sonographer measured, which back-calculates the same EDD.

It is an estimate. Only about 5% of babies arrive on the exact EDD, and a normal arrival window spans weeks 37–42 of pregnancy. Individual variation in ovulation timing, implantation, and fetal growth means no formula can pinpoint a single correct date. A first-trimester ultrasound typically narrows the window more than a period-based calculation alone.

Completely free. No sign-up, no downloads, and no limits — just enter a date and get your result instantly.

You can enter your cycle length in the LMP method and the tool shifts the EDD by the difference from 28 days. A 30-day cycle, for example, moves the EDD two days later; a 25-day cycle moves it three days earlier. This adjustment reflects that longer or shorter cycles shift ovulation relative to the period start date.

Yes. Select the "Conception Date" method to add 266 days from the date of conception or ovulation. Select "Ultrasound" to enter the scan date and the gestational age your sonographer measured — the tool back-calculates the EDD from those two pieces of information.

A first-trimester ultrasound (before about 13–14 weeks) is generally more accurate than period-based dating, because it directly measures fetal size rather than assuming ovulation timing. Clinically, when an ultrasound EDD and an LMP-based EDD differ by more than about 7 days, the ultrasound date takes precedence (per ACOG guidelines).

The tool shows your current gestational age in weeks and days — calculated from the effective last menstrual period date to today — along with which trimester you are in. Trimester one covers weeks 1–13, trimester two covers weeks 14–27, and trimester three covers week 28 onward.

No. All date calculations happen entirely in your browser. Your dates are never stored or sent to a server, so your information stays completely private.

Yes. The tool is fully responsive — inputs and results stack vertically on small screens so everything remains readable and tap-friendly without horizontal scrolling.

No. The calculator provides a general planning estimate, not medical advice. Please begin prenatal care early — your doctor or midwife will confirm your dating (usually with a first-trimester ultrasound) and guide your care throughout the pregnancy.