How an estimated due date is calculated
This calculator estimates a due date from the first day of the last menstrual period (LMP), with an adjustment for the cycle length you enter. The standard calendar method counts 280 days (40 weeks) from the first day of an LMP and assumes a typical 28-day cycle. A date from this tool is an estimate, not a prediction of the day birth will occur.
Why clinical dating may differ
The LMP may be uncertain, cycles may be irregular, and ovulation timing varies. The American College of Obstetricians and Gynecologists explains that ultrasound in the first trimester is the most accurate method to establish or confirm gestational age in many pregnancies. See its guidance on estimating the due date.
Use this result for general calendar reference only. A prenatal care professional can review the dates and confirm the estimate based on the pregnancy’s history and clinical information.
Tips for accurate results
A standard estimate adds 280 days, or 40 weeks, to the first day of the last menstrual period, a method known as Naegele's rule. If your cycle is much longer or shorter than 28 days or you know the conception date, the estimate can be adjusted. An early ultrasound may refine the date.
Common mistakes and limits
Only a small fraction of babies arrive on the due date, and a normal pregnancy can end several weeks either side of it. Your healthcare provider's dating takes priority over any online tool. Contact your provider promptly with any concerns about your pregnancy.