What is the estimated date of confinement (due date) for a pregnant patient whose last menstrual period was on [DATE]?

Calculating the Estimated Due Date (EDC) from Last Menstrual Period

To calculate the estimated date of confinement (EDC) from the last menstrual period (LMP), add 280 days (40 weeks) to the first day of the LMP, which is equivalent to adding 9 months and 7 days using Naegele's rule. 1, 2

Standard Calculation Method

• Naegele's Rule: Add 280 days (40 weeks) to the first day of the last menstrual period to determine the EDC 2
• This assumes a standard 28-day menstrual cycle with ovulation occurring around day 14 1
• The calculation can be simplified as: LMP + 9 months + 7 days 2

Critical Adjustments for Irregular Cycles

If the patient has irregular cycles (e.g., 35-day cycle instead of 28-day), you must adjust the calculation by adding the difference in cycle length to the standard EDC. 1

• For a 35-day cycle: Add 7 extra days to the standard calculation (35 - 28 = 7 days) because ovulation occurs around day 21 instead of day 14 1
• This adjustment accounts for the delayed ovulation that occurs with longer cycles 1

Why Accurate Dating Matters Clinically

• Incorrect dating directly affects prenatal screening interpretation, as maternal serum AFP levels increase 10-15% per week in the second trimester 1
• Inaccurate EDC can lead to inappropriate timing of interventions and misinterpretation of screening results 1
• The timing of delivery management decisions depends critically on accurate gestational age assessment 1

Optimal Dating Strategy Going Forward

First-trimester ultrasound with crown-rump length (CRL) measurement between 8-13 weeks provides superior accuracy (±5-7 days) compared to LMP-based dating and should be obtained when possible. 1

• When CRL and LMP dating differ by ≥5 days, adopt the CRL-based gestational age for all clinical decision-making 1
• CRL measurement is more accurate than mean sac diameter as soon as an embryo is measurable 3
• Gestational age is conventionally reported as completed weeks from the first day of the LMP 3

Important Caveats

• Naegele's rule should be considered a guideline, not a definite date, as multiple factors influence pregnancy length including ethnicity, height, menstrual cycle variations, parity, and maternal weight 2
• The 40-week calculation applies to the majority but not all pregnancies 2
• Many women incorrectly view 40 weeks as a "deadline," which can create unnecessary anxiety 2