The expected due date is estimated from the first day of the last menstrual period — pregnancy lasts on average 280 days (40 weeks) from that date.

How the calculation works

The calculator applies the medically used Naegele’s rule: due date = first day of the last period + 280 days, adjusted if the cycle is longer or shorter than 28 days. The result is an estimate — only about 5% of births happen exactly on the calculated day, and an ultrasound with your doctor remains the most accurate dating.