Calculating Gestational Age from Last Menstrual Period
To calculate gestational age, count the number of complete weeks from the first day of the last menstrual period (LMP) to today's date, expressing the result as weeks and days (e.g., 15 weeks 5 days) rather than rounded weeks. 1
Standard Calculation Method
Use Naegele's rule as the foundation: Add 280 days (40 weeks) to the first day of the LMP to determine the estimated due date, then calculate backward from today to determine current gestational age 1, 2
Express gestational age precisely as weeks and days (e.g., 12 weeks 5 days) or decimal weeks (12.7 weeks) rather than rounded weeks for optimal clinical accuracy 3, 1
The standard calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14 4
Adjustments for Irregular Cycles
For cycles longer than 28 days, add the difference between the patient's cycle length and 28 days to the calculated gestational age 2
- Example: For a 35-day cycle, add 7 days to the standard calculation since ovulation typically occurs around day 21 rather than day 14 2
For cycles shorter than 28 days, subtract the difference from the calculated gestational age 4
Women with long menstrual cycles are frequently misclassified as postterm when standard LMP dating is used without adjustment 4
Critical Limitations and Verification Needs
LMP-based dating has significant limitations: It assumes accurate recall, regular cycles, and that the last bleeding episode was true menses rather than implantation bleeding or other causes 5, 6
The positive predictive value of LMP estimates decreases dramatically from term deliveries (94.9%) to preterm (77.5%) to postterm (11.9%) 6
First-trimester ultrasound with crown-rump length (CRL) measurement between 8-13 weeks provides superior accuracy (±5-7 days) compared to LMP dating and should be obtained when possible 1, 2
When a discrepancy of ≥5 days exists between LMP and first-trimester CRL dating, adopt the CRL-based gestational age for all clinical decision-making 1, 2
Clinical Implications of Accurate Dating
Accurate gestational age is essential for appropriate timing of prenatal screening tests, as maternal serum AFP levels increase 10-15% per week in the second trimester 1, 2
Incorrect dating can lead to inappropriate timing of interventions, unnecessary inductions, and increased cesarean section rates 2, 6
Record both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date for clinical decisions when available 1