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.

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.