Pregnancy & Due Date Tracker
Estimate a due date and current week of pregnancy based on the first day of the last menstrual period.
How the estimate is calculated
The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle — which is why it's called an estimated due date, not a fixed one.
Why so few births land exactly on the date
Full-term birth is considered a range, roughly 37–42 weeks, and cycle length variation or irregular periods can shift the real date from the calculated one. An early ultrasound typically refines this estimate more accurately than LMP-based dating alone.