Calculating Gestational Age from Last Menstrual Period
Gestational age is calculated by counting the number of completed weeks and days from the first day of the last menstrual period (LMP) to today's date. 1, 2
Standard Calculation Method
Add 280 days (40 weeks) to the first day of the LMP to determine the estimated due date, then count backward from that date or forward from the LMP to establish current gestational age. 2, 3
Express gestational age as completed weeks and days (e.g., 15 weeks and 5 days should be reported as "15 weeks 5 days" or "15.7 weeks" in decimal format), never as rounded weeks, as this improves accuracy for clinical decision-making. 1, 3
The standard calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. 4
Adjustments for Irregular Cycles
For women with cycles longer than 28 days, add the difference between their cycle length and 28 days to the standard due date calculation. 2
For example, a woman with a 35-day cycle ovulates approximately 7 days later than the standard assumption, requiring an additional 7 days to be added to the calculated due date. 2
Women with longer menstrual cycles are frequently misclassified as post-term when using standard LMP dating without adjustment. 4
Critical Limitations of LMP Dating
LMP-based dating should be replaced by first-trimester ultrasound crown-rump length (CRL) measurement whenever a discrepancy of ≥5 days exists between the two methods. 2, 3
First-trimester ultrasound (8-13 weeks) provides accuracy within ±5-7 days compared to ±8-14 days for LMP dating, making it the gold standard for pregnancy dating. 3, 5
LMP dating is less accurate in young women, non-Hispanic Black and Hispanic women, women of non-optimal body weight, and mothers who deliver low-birthweight infants. 5
On average, LMP-based estimates are approximately 0.8-1 day longer than ultrasound estimates, with women tending to overestimate gestational age by recall. 5, 6
Clinical Implications
Accurate dating is essential because maternal serum AFP levels increase by 10-15% per week in the second trimester, and even small dating errors can lead to misinterpretation of prenatal screening results. 1, 2, 3
Never average LMP and ultrasound dates—always adopt the ultrasound-based gestational age when first-trimester CRL measurement is available. 3
Document both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound date for all clinical decisions when available. 3
When Ultrasound Dating Should Be Obtained
First-trimester ultrasound with CRL measurement between 8-13 weeks should be performed to establish the most accurate gestational age, particularly when LMP is uncertain, irregular cycles are present, or discrepancies exist. 2, 3
For pregnancies at ≥14 weeks when LMP is uncertain, biparietal diameter (BPD) measurement can be used for dating and has the added benefit of ruling out anencephaly. 1, 3