Calculating Age of Gestation from Last Menstrual Period
To calculate gestational age from LMP, 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) rather than rounded weeks. 1
Standard LMP-Based 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 work backward from today's date to calculate current gestational age 2
For women with regular 28-day cycles, the standard calculation assumes ovulation occurs on day 14, making the calculation straightforward 2, 3
For women with irregular or non-28-day cycles, adjust the calculation by adding the difference between the patient's cycle length and 28 days 2
- Example: For a 35-day cycle, ovulation typically occurs around day 21 (a 7-day delay), so add 7 days to the standard due date calculation 2
Critical Limitations and When to Abandon LMP Dating
LMP-based dating should be replaced by first-trimester ultrasound when available, as crown-rump length (CRL) measurement between 8-13 weeks provides accuracy within ±5-7 days compared to ±11 days for LMP 1, 4
When a discrepancy of ≥5 days exists between LMP and first-trimester CRL dating, immediately adopt the CRL-based gestational age for all clinical decision-making and discard the LMP-based calculation 1, 5
LMP calculations systematically overestimate gestational age in women with longer menstrual cycles, leading to false classification of post-term pregnancies (up to 14.6% postterm births in women with long cycles vs. 0% in those with short cycles) 3
Documentation Requirements
Record both the LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date for all clinical decisions when first-trimester ultrasound is available 1, 5
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
Common Pitfalls to Avoid
Never average LMP and ultrasound dates - always prioritize the ultrasound date when first-trimester CRL is available 1, 5
Avoid using manual gestational age wheels, as they demonstrate significant bias with individual differences of up to 4 days compared to accurate 280-day calculations; electronic calculators are recommended instead 6
Be aware that LMP reliability varies by maternal characteristics - discrepancies between LMP and ultrasound are greater among young women, non-Hispanic Black and Hispanic women, women of non-optimal body weight, and mothers of low-birthweight infants 4
Clinical Importance of Accurate Dating
Accurate dating is crucial because maternal serum AFP levels increase 10-15% per week in the second trimester, and even small dating errors can lead to misinterpretation of screening results 1, 2
Incorrect dating affects the appropriate timing of Down syndrome screening, neural tube defect screening, corticosteroid administration (24-34 weeks), and magnesium sulfate for neuroprotection (before 32 weeks) 1, 5