What is the gestational age given the last menstrual period (LMP) date?

Calculating Gestational Age from Last Menstrual Period

To calculate gestational age from the LMP date, count the number of completed weeks and days from the first day of the last menstrual period to today's date, expressing the result as weeks and days (e.g., 15 weeks 5 days). 1

Standard Calculation Method

• Add 280 days (40 weeks) to the first day of the LMP to determine the estimated due date, though this assumes a standard 28-day menstrual cycle with ovulation at day 14. 1, 2

• Express gestational age as weeks and days (e.g., 15 weeks 5 days) or decimal weeks (15.7 weeks) rather than rounded weeks for optimal accuracy. 1

• The first trimester is defined as gestational age ≤13 weeks 6 days. 3

Critical Adjustments for Irregular Cycles

• For patients with cycles longer than 28 days, add the difference between the patient's cycle length and 28 days to the standard calculation. For example, with a 35-day cycle, add 7 days to the due date (ovulation occurs around day 21 instead of day 14). 4

• Women with longer menstrual cycles have a higher proportion of falsely classified post-term deliveries when using standard LMP dating without adjustment. 5

Important Limitations of LMP-Based Dating

• LMP-based dating should be replaced by first-trimester ultrasound crown-rump length (CRL) measurement whenever available, as ultrasound is more accurate with precision within ±5-7 days compared to LMP's typical error of 8-9 days. 1, 2

• When a discrepancy of ≥5 days exists between LMP and CRL dating in the first trimester, adopt the CRL-based gestational age for all clinical decision-making. 1, 6

• LMP-based estimates are less reliable in young women, non-Hispanic Black and Hispanic women, women of non-optimal body weight, and may be affected by uncertainty about the LMP date or bleeding not associated with menses. 7, 8

Documentation Requirements

• Record both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date for clinical decisions when available. 1, 6

• Never average LMP and ultrasound dates—always prioritize the ultrasound date when obtained in the first trimester. 6, 4

Clinical Significance of Accurate Dating

• Accurate dating is crucial for appropriate timing of Down syndrome and neural tube defect screening, as maternal serum AFP levels increase by 10-15% per week in the second trimester. 1, 4

• Incorrect dating can lead to misinterpretation of screening results and inappropriate timing of interventions. 4