EDD Calculator - Estimated Date of Delivery

Pregnancy dating relies on standard obstetrical rules, primarily Naegele's rule adjusted for a typical 280-day gestation period (40 weeks). When using your last menstrual period, the formula calculates the estimated delivery date as: eddP = LMP + 7 days - 90 days + 365 days (or simply adding 280 days to the LMP). Alternatively, when utilizing ultrasound measurements, the tool adjusts the delivery date based on the exact gestational age observed at a specific scan date using: eddUS = USdate + (281 - (USweeks * 7 + USdays)) + period adjustment. The calculator also determines current gestational age and compares ultrasound versus LMP dating discrepancies to give you a comprehensive timeline.

Imagine an expectant mother whose Last Menstrual Period (LMP) started on January 1, 2024. To find her estimated delivery date using the LMP method, we add 280 days (or apply Naegele's rule by adding 7 days to get January 8, subtracting 3 months to October 8, and adding a year): January 1, 2024 + 280 days = October 7, 2024. If she later has an ultrasound on April 1, 2024, showing the fetus measuring precisely 12 weeks and 3 days (87 days), the ultrasound dating equation calculates the expected arrival date based on those precise anatomical measurements, allowing her doctor to compare the two metrics for maximum accuracy.

First trimester ultrasounds are generally considered the gold standard for dating a pregnancy because early fetal growth rates are remarkably uniform across all pregnancies. If your menstrual cycles are irregular or longer than the standard 28 days, rely primarily on early ultrasound dating rather than your LMP to avoid miscalculating gestational age. Always consult your obstetrician or midwife to interpret these dates in the context of your overall health and clinical history.