What is the estimated date of delivery if the last normal menstrual period (LMP) is on a given date?

Estimated Date of Delivery Calculation

To calculate the estimated due date (EDD) 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

• Add 280 days to the first day of the LMP to determine the EDD, assuming a regular 28-day menstrual cycle 1, 2
• Express the result as a specific date (month/day/year) rather than just weeks of pregnancy 3
• This calculation assumes ovulation occurred on day 14 of a 28-day cycle 1

Adjustments for Irregular Cycles

If the patient has cycles longer or shorter than 28 days, you must adjust the calculation:

• Add the difference between the patient's cycle length and 28 days to the standard EDD 1
• For example, with a 35-day cycle, add 7 additional days to the calculated EDD (35 - 28 = 7 days) 1
• This accounts for delayed ovulation in longer cycles, which typically occurs around day 21 in a 35-day cycle rather than day 14 1

Critical Limitations and Next Steps

LMP-based dating should be considered provisional and requires ultrasound confirmation:

• First-trimester ultrasound with crown-rump length (CRL) measurement between 8-13 weeks is the gold standard, providing accuracy within ±5-7 days compared to ±1-2 weeks for LMP dating 1, 3, 4
• When CRL and LMP dates differ by ≥5 days, adopt the CRL-based gestational age for all clinical decisions 1, 3
• Manual gestational age calculators (wheels) have significant inaccuracies of up to 4 days and should be avoided; use electronic calculators or add exactly 280 days 5

Why Accuracy Matters

• Maternal serum AFP levels increase 10-15% per week in the second trimester, making even small dating errors clinically significant for screening interpretation 1, 3
• Inaccurate dating affects the timing of all prenatal screening tests, potentially leading to false-positive or false-negative results 1, 3
• The number of postterm pregnancies decreases from 10.3% to 2.7% when ultrasound dating replaces LMP-based dating 4

Documentation Requirements

• Record both the LMP-based EDD and ultrasound-based EDD in the medical record, but use the ultrasound date for clinical decision-making once available 3
• Express gestational age as weeks and days (e.g., 15 weeks 5 days) rather than rounded weeks for optimal precision 3