Pregnancy Due Date Calculator

About Due Date Calculations

A pregnancy due date is typically calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a standard 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, the due date is adjusted accordingly. For example, a 32-day cycle would shift the estimated due date forward by 4 days, as ovulation likely occurs later.

When calculating from a known conception date, the due date is 266 days (38 weeks) from conception. This is because the standard 40-week gestational timeline includes approximately two weeks before ovulation and conception actually occur. For IVF patients, the calculation accounts for the embryo's age at transfer—a 5-day blastocyst transfer means the embryo is already 5 days past conception, so 261 days are added to the transfer date.

It is important to understand that a due date is an estimate, not a guarantee. Only about 5% of babies are born on their exact due date. Most healthy pregnancies result in delivery between 37 and 42 weeks of gestation. The three trimesters divide pregnancy into roughly equal periods: the first trimester (weeks 1-12) involves rapid early development, the second trimester (weeks 13-27) is often called the most comfortable period, and the third trimester (weeks 28-40) involves final growth and preparation for birth.

This calculator is intended for informational purposes only and is not a substitute for medical advice. Always consult with your healthcare provider for accurate dating of your pregnancy, which may include ultrasound measurements that can provide more precise estimates than date-based calculations alone. Your provider may adjust your due date based on first-trimester ultrasound findings.