Due Date Calculator

A due date is a convention, not a prediction to the day. Clinicians count 280 days from the first day of the last period because, on average, ovulation comes about two weeks after a period starts and a pregnancy lasts about 38 weeks from there. The calculator does the same sum, then moves the date by the days your cycle is longer or shorter than 28, because ovulation sits about two weeks before the next period rather than on a fixed day 14.

Babies keep their own schedule. In an Australian study of 18,708 births where labour started on its own, 5% arrived on the estimated date and about two thirds within seven days of it. Even when the day of ovulation was measured with hormone tests, healthy pregnancies varied in length by 37 days (Jukic et al.). That is why the result shows the term window, from 37 weeks to the end of 41 weeks, as prominently as the date itself.

An IVF transfer gives the most exact date, because the embryo’s age and the day it was placed are known. A dating scan in the first trimester comes next: measuring the embryo up to 13 weeks 6 days is accurate to about 5 to 7 days either way, and ACOG calls it the most accurate way to set or confirm the date. A known conception date is good when you tracked ovulation. The last period is the method everyone can use, but it depends on remembering the date and on ovulation happening when the average says; ACOG notes that only about half of people recall it accurately.

When an early scan and the last period disagree, the scan wins once the difference is bigger than the limits below. Your clinician makes that call and records one date for the whole pregnancy; if theirs differs from this page, plan with theirs.

Pregnancy weeks start on the first day of the last period, about two weeks before conception, so at “4 weeks” the embryo is about two weeks old. A scan or an IVF date is turned into the same count so every method speaks one language. The first trimester runs to the end of week 13, the second from 14 weeks to the end of 27, and the third from 28 weeks on. From 37 weeks a birth is called early term, from 39 full term, from 41 late term and from 42 postterm, the labels ACOG and the Society for Maternal-Fetal Medicine use.

Everything here is worked out in your browser. The dates you type are not sent anywhere, not saved and not put in the address bar, so they don’t end up in your history or in a link by accident. If you want to send someone your dates, Share asks first and shows you the link before anything goes. There are no accounts, no ads and no app to download, and closing the page clears it.