Estimate a due date from the first day of the last menstrual period, adjusted for the average cycle length you enter.
Your Expected Due Date
How the Estimated Due Date Is Calculated
The calculator starts with the first day of the last menstrual period (LMP) and adds 280 days (40 weeks). It adjusts that estimate by the difference between the entered cycle length and a 28-day reference cycle. This is a calendar-based estimate and assumes the entered date and cycle length are accurate.
Why a due date is only an estimate
Ovulation and conception do not occur on the same cycle day for everyone. Cycle irregularity and clinical dating can change the estimate. A healthcare professional may revise pregnancy dating using medical history and ultrasound findings.
Health note: This tool cannot confirm pregnancy, gestational age, or fetal health. Use prenatal care for clinical dating and medical decisions.
Source and important context
The American College of Obstetricians and Gynecologists explains that pregnancy dating may use the last menstrual period and ultrasound; first-trimester ultrasound is the most accurate method to establish or confirm gestational age. ACOG: Methods for Estimating the Due Date. This page provides general educational estimates and is not a diagnosis or a substitute for individualized care. Page information updated: 4 November 2026.