Published July 28, 2026
How Accurate Are Due Date Calculators, Really?
Due date calculators are useful planning tools, but it's worth understanding exactly what they can and can't tell you — and why "due date" is really shorthand for "estimated date," not a guarantee.
The two common calculation methods
The most widely used method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle and ovulation on day 14. If you know your conception date more precisely (for example, from fertility tracking or IVF), a conception-based calculation tends to be slightly more precise since it removes the cycle-length assumption.
Why ultrasound dating is considered more accurate
An early first-trimester ultrasound (typically between weeks 8-13) measures the size of the fetus and is generally considered the most accurate dating method, because it's based on actual observed development rather than an assumption about when you ovulated. If your care provider's ultrasound-based due date differs from a calendar-based estimate by more than about 5-7 days, your provider will usually use the ultrasound date going forward.
Only about 5% of babies arrive on their exact due date
Full-term is generally considered anywhere from 37 to 42 weeks, and the vast majority of babies are born somewhere in that window — not on the specific calculated date. This is completely normal and expected; the due date is better understood as the midpoint of a likely range than a deadline.
What actually affects accuracy
- Irregular cycles: Calendar-based estimates assume a standard 28-day cycle; longer or shorter cycles shift the real ovulation date and therefore the estimate.
- Uncertain last period date: If you're not sure of the exact first day of your last period, the calculation starts from an already-uncertain point.
- First pregnancy vs. subsequent pregnancies: Some research suggests slight average differences in typical gestation length between first and later pregnancies, though individual variation is large.
Use it as a planning tool, not a deadline
Our due date calculator gives you a solid starting estimate for planning — for appointments, leave, and preparation — but your care provider's ongoing measurements are what actually refine the picture as your pregnancy progresses.