Pregnancy Due Date Calculator
Estimate your baby's due date based on the first day of your last menstrual period (LMP) using Naegele's rule. This widely-used method calculates the due date as 280 days (40 weeks) from your LMP, with an adjustment for cycle length.
Naegele's rule, developed by German obstetrician Franz Naegele in 1812, is the standard method for estimating pregnancy due dates. The formula adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle. For women with longer or shorter cycles, the estimated date is adjusted accordingly, since ovulation typically occurs 14 days before the next period. While widely used, only about 5% of babies are born on their exact due date. A full-term pregnancy ranges from 37 to 42 weeks. First-trimester ultrasound is considered the most accurate method for pregnancy dating and may adjust the due date if it differs significantly from the LMP-based estimate.