Given a last menstrual period (LMP) of [DATE], what is the estimated due date (EDD)?

Calculating Your Estimated Due Date (EDD)

Add 280 days (40 weeks) to the first day of your last menstrual period to calculate your estimated due date. 1

The Standard Calculation Method

• Use Naegele's rule: Add 280 days to the first day of your LMP, which is the standard method recommended by the American College of Obstetricians and Gynecologists 1
• Alternatively, you can calculate this as: add 9 months and 7 days to the first day of your LMP 2
• Gestational age is always calculated from the first day of the last menstrual period, not from conception 1

Important Considerations About This Calculation

• This assumes a 28-day menstrual cycle with ovulation on day 14 3
• If you have irregular cycles (for example, a 35-day cycle), you need to adjust by adding the difference between your cycle length and 28 days to the standard calculation 3
• For a 35-day cycle, ovulation typically occurs around day 21 (7 days later than standard), so add 7 days to your calculated EDD 3

Why Accurate Dating Matters Critically

• Incorrect dating affects the timing of all prenatal screening tests, with maternal serum AFP levels increasing 10-15% per week in the second trimester, potentially leading to misinterpretation of screening results 3
• Accurate dating is essential for timely performance of Down syndrome and neural tube defect screening 4
• The timing of interventions throughout pregnancy depends on correct gestational age 3

The Gold Standard: First-Trimester Ultrasound

• First-trimester ultrasound with crown-rump length (CRL) measurement between 8-13 weeks provides accuracy within ±5-7 days and is superior to LMP-based dating 4, 3
• At 7.5 weeks, CRL measurement provides estimates within approximately ±1.5 days of actual gestational age 4
• When a discrepancy of ≥5 days exists between LMP and CRL dating, adopt the CRL-based gestational age for all clinical decision-making 3
• Always use the ultrasound-based date as the official pregnancy date, even when it matches your LMP, because ultrasound is inherently more accurate 4

Common Pitfalls to Avoid

• Do not use manual gestational age wheels or calculators - they have been shown to have significant bias with individual differences of up to 4 days, and their accuracy varies throughout the year 5
• Electronic devices are recommended as they consistently have no error 5
• Do not average the LMP and ultrasound dates - always use the ultrasound date when available in the first trimester 4
• Do not adjust the gestational age based on later ultrasounds if first-trimester dating was accurate 4

Reporting Gestational Age Correctly

• Express gestational age in weeks and days (e.g., 7 weeks 5 days) instead of rounded weeks to optimize accuracy in clinical decision-making 4
• Document both the LMP-based and CRL-based dates in the medical record, but use the ultrasound-based date for all clinical decisions 4

Reality Check on the 40-Week Standard

• The median pregnancy length based on ultrasound dating is actually 283 days (not 280), with 22.9% of women still pregnant 7 days after the EDD 6
• Multiple factors influence actual pregnancy length, including ethnicity, height, menstrual cycle variations, timing of ovulation, parity, and maternal weight 2
• Naegele's rule should be considered a guideline for the expected date, not a definite date 2