Pregnancy Due Date Calculator: How Accurate Is It?

A pregnancy due date calculator estimates when your baby will arrive based on the first day of your last menstrual period (LMP). Most calculators use Naegele's Rule: add 7 days to the LMP, then subtract 3 months and add 1 year.

How Accurate Is It?

Only about 4–5% of babies are born exactly on their due date. Most deliveries occur within a 2-week window before or after the estimated date. This is why doctors often refer to it as an "estimated date of delivery" (EDD) rather than a guaranteed date.

When Is an Ultrasound More Accurate?

A first-trimester ultrasound (between weeks 8 and 13) is the most accurate way to date a pregnancy. If your ultrasound EDD differs from your LMP-based EDD by more than 5–7 days, your doctor will usually adjust the due date to match the ultrasound.

Factors That Can Shift the Due Date

• Irregular cycles
• Not knowing the exact LMP
• Conception occurring later than expected
• Assisted reproductive technologies (e.g., IVF)

What "Full Term" Really Means

Doctors now use these categories instead of a single due date:

• Early term: 37–38 weeks
• Full term: 39–40 weeks
• Late term: 41 weeks
• Post term: 42 weeks or more

Calculate your estimated due date with our free due date calculator.