Understanding the Due Date Calculator
A due date calculator estimates when your baby is likely to arrive, giving you an anchor point for prenatal appointments, planning, and tracking your pregnancy week by week. Most estimates use Naegele’s rule, which counts 280 days (40 weeks) from the first day of your last menstrual period — the same method most providers use at your first visit.
Why it is an estimate, not a deadline
Only about one in twenty babies actually arrives on the predicted due date. A full-term birth can happen anytime between 37 and 42 weeks, so the date is best thought of as the middle of a likely window rather than a fixed appointment. Your provider may refine the estimate with an early ultrasound, which measures the baby directly and can be more accurate than the last-period method.
How cycle length affects the date
The standard calculation assumes a 28-day cycle with ovulation around day 14. If your cycles are consistently longer or shorter, ovulation — and therefore conception — shifts, which can move your true due date by several days. If you know your cycle length or conception date, using it produces a more personalized estimate.
Tips & things to know
- •An early ultrasound is usually the most accurate way to date a pregnancy.
- •Babies are considered full term anywhere from 37 to 42 weeks.
- •Track from the first day of your last period for the standard estimate.