Pregnancy Due Date Calculator

The day you find out you are pregnant, the first question everyone asks is: when is the baby due? This calculator answers with Naegele's rule, the standard obstetric method: add 280 days — 40 weeks — to the first day of your last menstrual period. Enter the year, month, and day of that period plus your average cycle length, and the tool returns your estimated due date along with the approximate conception day (usually about two weeks after the period started).

Take a last period beginning January 15, 2025, with a 28-day cycle. The calculator counts forward 280 days to land on October 22, 2025, and estimates conception around January 29. Longer or shorter cycles nudge the date, because ovulation — and therefore conception — shifts with cycle length. The tool also shows gestation days elapsed, which is the number your OB's office will ask about at every appointment.

Here is the reality check every parent learns: only about 5 percent of babies arrive on their due date. First babies often run late, and your provider will refine the date with an early ultrasound, which beats calendar math. This calculator is for planning and excitement, not medical dating — your clinician's estimate always wins.