Pregnancy Due Date Calculator

How the Pregnancy Due Date Calculator Works — and How Accurate It Really Is

The moment you find out you're pregnant, one of the first questions is obvious: when is the baby due? This calculator gives you an estimate based on a simple, centuries-old formula — but the story behind that formula, and the reasons why a due date is really a “due window,” is worth understanding. It also shows your current gestational age, which is what your doctor will use at every prenatal visit to track how the pregnancy is progressing.

The formula behind it

The standard method is called Naegele's rule, named after the German obstetrician Franz Naegele who popularized it in the early 1800s. The rule is simple: take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that's your estimated due date. In practice, most doctors calculate it a slightly simpler way: add 7 days to the LMP, then subtract 3 months, then add 1 year. So if your LMP was on March 10, 2025, your estimated due date would be December 17, 2025. This 280-day figure isn't random — it's based on the observation that human pregnancies last about 40 weeks from the last period, which works out to roughly 38 weeks from actual conception (since ovulation usually happens about two weeks after the LMP begins).

Gestational age vs. fetal age

Here's a distinction that confuses a lot of expecting parents: there are two ways to count how far along a pregnancy is. Gestational age is counted from the first day of your last menstrual period, which is the standard used by doctors, ultrasound machines, and pregnancy apps everywhere. Fetal age (sometimes called embryonic age) is counted from the actual moment of conception, which is usually about two weeks later. So when your doctor says you're “8 weeks pregnant,” they mean 8 weeks of gestational age — the embryo itself is only about 6 weeks old. This is not a mistake or a shortcut; it's the universal convention, and it exists because it's very difficult to know the exact moment of conception, while the first day of the last period is usually known.

Why a due date is really a due window

Only about 4% of babies are born on their exact estimated due date. The vast majority arrive within two weeks before or after it. A pregnancy is considered “full term” between 39 and 40 weeks, “early term” between 37 and 38 weeks, “late term” between 41 and 42 weeks, and “post term” beyond 42 weeks. Anything before 37 weeks is considered preterm. Doctors use the due date as the center of a window rather than a specific date, and they often adjust it after an early ultrasound, which can measure the embryo directly and give a more accurate estimate than the LMP-based formula. If your cycle is irregular, if you're unsure of your LMP, or if you conceived through IVF, the due date may shift by a week or more.

Limitations to keep in mind

This calculator assumes a regular 28-day menstrual cycle with ovulation on day 14, which is the textbook scenario but not the norm for every woman. If your cycle is shorter or longer, your actual due date may differ by several days. The calculator also can't account for factors like multiple pregnancies (twins often arrive earlier), first-time vs. subsequent pregnancies, or pre-existing medical conditions — all of which can affect timing. Most importantly, this tool is for educational and planning purposes only. It is not a substitute for prenatal care. Your healthcare provider will confirm your due date through examination and imaging, and their estimate is the one to follow for medical decisions.