What is the estimated due date (estimated date of confinement) when the last menstrual period (LMP) is [date]?

Calculating Estimated Due Date from Last Menstrual Period

Add 280 days (40 weeks) to the first day of the last menstrual period to determine the estimated date of confinement (EDC), which is calculated as 9 months plus 7 days from the LMP. 1

Standard Calculation Method

• Gestational age is calculated from the first day of the last menstrual period, with the EDC occurring at 40 weeks (280 days) from that date. 2

• The traditional method, known as Naegele's rule, adds 9 months and 7 days to the first day of the LMP to arrive at the expected delivery date. 1

• Gestational age should be reported as number of weeks and number of days (e.g., 28 weeks 3 days), not as rounded weeks. 2

Important Considerations for Accuracy

• Early ultrasound dating (particularly first-trimester crown-rump length measurement) is more accurate than LMP dating alone and should be used to confirm or adjust the EDC when available. 3

• LMP-based dating assumes a regular 28-day menstrual cycle with ovulation occurring on day 14, but variations in cycle length, timing of ovulation, maternal age, ethnicity, height, parity, and weight can all affect actual pregnancy duration. 1

• When LMP dates are uncertain or unknown, ultrasound biometry performed before 22 weeks gestation provides the most reliable gestational age estimation. 3

Clinical Application Algorithm

• If the LMP date is certain and the patient has regular menstrual cycles: Calculate EDC by adding 280 days to the first day of LMP. 2, 1

• If first-trimester ultrasound is available: Use the ultrasound-derived gestational age as the primary dating method, as it is more accurate than LMP alone. 3

• If there is discrepancy between LMP and early ultrasound dating: Defer to the ultrasound dating when performed in the first trimester, as routine scanning in the first half of pregnancy has demonstrated clear advantages over menstrual dating. 3

Common Pitfalls to Avoid

• Do not rely solely on LMP dating when the date is uncertain, menstrual cycles are irregular, or the patient was recently using hormonal contraception, as this will lead to inaccurate gestational age assignment. 3

• Avoid treating the 40-week date as an absolute deadline, as normal pregnancy duration varies significantly among individuals, and Naegele's rule should be considered a guideline rather than a definite date. 1

• Do not use LMP dating for assisted reproductive technology pregnancies—these require specific calculations based on embryo transfer date and developmental stage. 4