Pregnancy Due Date
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About This Calculator
This calculator uses Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. Only 4-5% of babies are born on their exact due date. Most are born within 2 weeks before or after. Your healthcare provider may adjust your due date based on ultrasound measurements.
About the Pregnancy Due Date
The Pregnancy Due Date tool estimates your baby's expected arrival date, the centerpiece of prenatal planning. The standard method uses Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), reflecting the average length of a full-term pregnancy counted from that reference point.
Because pregnancy is conventionally dated from the LMP rather than conception, the estimated due date includes roughly two weeks before you actually conceived. If you know your conception or ovulation date instead, the tool can date from there by adding about 266 days. For people with cycles longer or shorter than the typical 28 days, due dates calculated from LMP can be off, which is why ultrasound dating in the first trimester is often more precise.
Expectant parents use this estimate to schedule prenatal appointments, plan maternity leave, track which week and trimester they are in, and prepare for the birth. It pairs well with an ovulation calculator for those still planning conception and with general date calculators for counting down the weeks remaining.
Keep in mind that only about one in twenty babies arrives exactly on the due date; most births occur within a window of two weeks before or after. Treat the result as a target around which to plan rather than a fixed deadline, and always confirm dating with your healthcare provider, especially if your cycle is irregular.
Frequently asked questions
- How is a due date calculated from the last period?
- Using Naegele's rule, you add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a typical 28-day cycle with ovulation around day 14.
- Why is pregnancy dated from the last period and not conception?
- The last menstrual period is an easy date to remember and observe, whereas conception is harder to pinpoint. Dating from LMP adds about two weeks before actual conception by convention.
- How accurate is the estimated due date?
- It is an estimate; only about 5 percent of babies arrive exactly on the predicted date. Most births occur within two weeks before or after the due date.
- What if my menstrual cycle is irregular?
- LMP-based dating can be less accurate with irregular or non-28-day cycles. An early ultrasound provides more reliable dating, so confirm with your provider.