Due Date Calculator

The due date is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If using conception date, add 266 days (38 weeks). Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after.

First trimester: weeks 1-12 (major organ development). Second trimester: weeks 13-27 (movement, gender visible on ultrasound). Third trimester: weeks 28-40 (rapid growth, lung maturation). Each trimester is approximately 13-14 weeks.

LMP stands for Last Menstrual Period - the first day of your most recent menstrual period before becoming pregnant. This date is used because conception typically occurs about 2 weeks after LMP. Most pregnancy dates and gestational age are calculated from LMP.

A due date based on LMP has a margin of error of about 2 weeks. Only 4-5% of babies arrive on the exact due date. An ultrasound in the first trimester (before 14 weeks) is the most accurate method, with a margin of error of about 5-7 days.