Pregnancy Due Date Calculator

Due date calculations based on the last menstrual period (LMP) are estimates — only about 5% of babies arrive on their exact due date. About 80% of babies are born within 2 weeks of the estimated date. Ultrasound dating in the first trimester is generally more accurate than LMP-based calculations.

Naegele's Rule is the standard method for calculating due dates: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the due date is adjusted accordingly.

The first trimester (weeks 1-12) involves major organ development. The second trimester (weeks 13-26) is often the most comfortable, with the baby growing rapidly. The third trimester (weeks 27-40) prepares for birth, with the baby gaining weight and the lungs maturing.