How Your Due Date Is Calculated (And Why It's Rarely Exact)

A due date is one of the first pieces of information expecting parents get, but it's worth understanding that it's a statistical estimate, not a prediction of the exact day labor will start. Only a small percentage of babies are actually born on their calculated due date — most arrive within a two-week window on either side.

How the standard due date is calculated

The most common method, called Naegele's Rule, estimates a due date 40 weeks from the first day of your last menstrual period (LMP) — not from the date of conception. The formula is:

This might seem odd, since pregnancy doesn't technically begin until conception (usually around two weeks after the LMP, at ovulation). But because the exact date of conception is rarely known with certainty, using the LMP as the reference point gives doctors a consistent, easily-recalled starting point that has been used for over a century.

Why the LMP method isn't perfectly precise

Naegele's Rule assumes a textbook 28-day cycle with ovulation on day 14. In reality, cycle length varies significantly between individuals — anywhere from about 21 to 35 days is considered normal — which shifts the actual conception date earlier or later than the formula assumes. This is one reason why an early ultrasound, which measures fetal size directly, is often considered more accurate than an LMP-based estimate, especially for people with irregular cycles.

The origin of Naegele's rule

The method most calculators use is called Naegele's rule: take the first day of the last menstrual period, add seven days and count back three months. It is named for the German obstetrician Franz Karl Naegele (1778 to 1851), who popularized it in the early nineteenth century, although similar approaches existed earlier. For example, a last period starting January 10, 2026 gives a due date of October 17, 2026. The rule assumes a 28-day cycle with ovulation on day 14 and a total of about 40 weeks from the last period, which is why it is less accurate for people with longer, shorter or irregular cycles.

How ultrasound changed dating

The first diagnostic obstetric ultrasound studies were published by Ian Donald and colleagues in Glasgow in the late 1950s, and ultrasound slowly became routine in prenatal care. In the first trimester, measuring the crown-rump length of the embryo gives a very accurate estimate of gestational age, usually within a few days. Doctors commonly adjust the due date when an early ultrasound and the last-period estimate differ by more than about a week, and early ultrasound dating is generally considered more reliable than the calendar method. Dating based on conception or IVF transfer dates can be even more precise.

What "due date" really means

Only about 4 to 5 percent of babies are born on their estimated due date. A normal pregnancy can deliver in a window of several weeks around it. The American College of Obstetricians and Gynecologists defines early term as 37 weeks to 38 weeks 6 days, full term as 39 weeks to 40 weeks 6 days, late term as 41 weeks to 41 weeks 6 days, and post-term as 42 weeks and beyond. Calling the due date a due window helps set realistic expectations.

Using the date wisely

• Use your estimated date to plan appointments, leave from work and the timing of prenatal tests.
• Do not treat the date as a deadline. Your care team decides when and how to monitor or induce if needed.
• Share your most recent menstrual period date and any early ultrasound results with your provider.

Our Pregnancy Due Date Calculator gives an estimate you can use as a starting point, and the Ovulation Calculator can help if you are tracking conception. Always follow your healthcare provider's guidance. This article is general information, not medical advice.

Tracking weeks of pregnancy

Pregnancy is measured in weeks starting from the LMP, not from conception — so by the time you find out you're pregnant, you're typically already considered "4 weeks" along, even though conception happened only about two weeks earlier. The full pregnancy is generally divided into three trimesters:

• First trimester: Weeks 1–13
• Second trimester: Weeks 14–27
• Third trimester: Weeks 28–40+

Why so few births happen exactly on the due date

Research consistently shows that only a small fraction of babies (commonly cited around 5%) are born on their exact calculated due date. A full-term pregnancy is generally considered anywhere from 37 to 42 weeks, meaning the "normal" window spans about five weeks — the single due date is really just the statistical midpoint of that range, not a firm deadline.

This calculator provides general informational estimates and is not a medical diagnosis. Always confirm your due date and pregnancy timeline with a qualified healthcare professional.