Pregnancy Due Date Calculator

The most common method uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your average cycle is longer or shorter than 28 days, the calculation is adjusted by adding or subtracting the difference. Alternatively, if you know your conception date, add 266 days (38 weeks). This calculator supports both methods (Source: American College of Obstetricians and Gynecologists, ACOG).

Date-based calculators provide a good estimate, but ultrasound dating in the first trimester (before 14 weeks) is generally more accurate, with a margin of error of about 5-7 days. Most babies are born within two weeks before or after their estimated due date. Full-term delivery ranges from 37 to 42 weeks of gestation. Your healthcare provider may adjust your due date based on early ultrasound measurements (Source: NICE Antenatal Care Guidelines).

Gestational age is measured from the first day of the last menstrual period (LMP), which is about two weeks before conception actually occurs. Fetal age (also called embryonic age) is measured from the actual date of conception. A full-term pregnancy is 40 weeks gestational age, but the baby has only been developing for about 38 weeks. Healthcare providers use gestational age as the standard measurement (Source: American Pregnancy Association).

Yes. Your healthcare provider may adjust your due date based on ultrasound measurements, especially if there is a discrepancy of more than 7 days between the ultrasound estimate and the LMP-based date. First-trimester ultrasounds are the most accurate for dating. If your cycles are irregular, ultrasound dating may be preferred over LMP-based calculations (Source: ACOG Committee Opinion on Methods for Estimating Due Date).