Due Date Calculator

This calculator estimates your due date using Naegele's rule, a method named after the 19th-century German obstetrician Franz Karl Naegele. The rule is simple: take the first day of your last menstrual period (LMP) and add 280 days, which is 40 weeks. That number comes from an old convention of counting pregnancy as ten lunar months of 28 days each, and it has remained the standard starting point for due date estimation ever since, even though modern obstetrics now leans on ultrasound measurements as a complementary check.

The single most common source of confusion with this method is what the 280 days actually represent. Pregnancy is dated from the first day of your last period, not from the day you actually conceived. Because ovulation and conception typically happen around two weeks after your period starts, your real gestational age from fertilization is usually about two weeks less than the "weeks pregnant" number your due date implies. In other words, when a due date calculator says you are at week 8, you have generally only been carrying the embryo for around six weeks since conception. This is a counting convention, not an error, but it trips up a lot of people the first time they encounter it.

Naegele's rule assumes a textbook 28-day cycle with ovulation occurring around day 14. Real cycles vary quite a bit from person to person, and even from month to month for the same person, which is why this calculator asks for your average cycle length rather than assuming 28 days for everyone. If your cycles run longer than 28 days, ovulation tends to happen later in the cycle, which pushes conception — and therefore the due date — later as well. A 35-day cycle, for example, shifts the estimated due date about a week later than the standard calculation would suggest. Shorter cycles work the other way, moving the estimate earlier. This adjustment brings the estimate closer to reality for people whose cycles don't match the 28-day textbook case, though it still can't fully correct for cycles that are irregular or unpredictable.

Once a due date is estimated, the rest of the pregnancy is described with two separate sets of milestones, both counted from that same last-period starting point. The first set is the trimesters: the American College of Obstetricians and Gynecologists (ACOG) places the first trimester through 13w 6d, the second from 14w 0d to 27w 6d, and the third from 28w 0d until birth. The second set describes the final five weeks, and it changed in 2013. Until then, every birth from 37 weeks onward was lumped together as "term". ACOG and the Society for Maternal-Fetal Medicine replaced that single label with four: early term (37w 0d to 38w 6d), full term (39w 0d to 40w 6d), late term (41w 0d to 41w 6d) and postterm (42w 0d and beyond). That is why this calculator gives you two end-of-pregnancy dates instead of one.

It's worth being clear-eyed about what a due date actually predicts, which is less than most people assume. Only a small share of babies are born on their exact estimated due date — most research suggests it's a low single-digit percentage. The much more useful way to think about a due date is as the midpoint of a range: most babies arrive somewhere within about two weeks before or after that date, and being born a bit early or a bit late within that window is entirely normal rather than a sign that something is wrong. Due dates are best understood as a planning estimate, not a deadline.

There are also situations where this last-period-based estimate is noticeably less reliable. If your cycles are irregular, if you're not sure exactly when your last period started, if you conceived while breastfeeding or shortly after stopping hormonal birth control, or if you have a condition that affects ovulation timing, the calculation here can be off by more than the usual margin. In these cases, and generally as pregnancy progresses, an early first-trimester ultrasound is often considered a more accurate way to date a pregnancy than menstrual history alone, because it measures the embryo or fetus directly rather than relying on an assumed cycle pattern. Your due date may be revised after that scan, and that's a normal part of prenatal care rather than something to worry about.

The most common way people misread these dates is to treat early term as the finish line and the due date as a deadline. It is neither. ACOG and the Society for Maternal-Fetal Medicine created the separate early-term label precisely because outcomes are not flat across those five weeks: adverse newborn outcomes, respiratory problems above all, are least frequent among uncomplicated pregnancies delivered between 39w 0d and 40w 6d. So the useful next step is not to circle the due date on a calendar. Write down both end-of-pregnancy dates this calculator gives you, take them to your next prenatal appointment, and ask what they mean for your own plan — particularly if a delivery date is being scheduled, and which side of 39 weeks it would fall on.