Management of 2-Week Discrepancy Between LMP and CRL Dating
When gestational age by LMP differs from CRL measurement by 2 weeks (≥14 days), you must reinterpret all screening test results using the ultrasound-based gestational age, and if the sample was drawn at less than 15 weeks, obtain a new sample with the corrected gestational age. 1
Primary Guideline for Dating Discrepancies
Adopt the CRL-based gestational age as the official gestational age for all clinical decision-making when there is a 2-week discrepancy. 1, 2 The American College of Medical Genetics explicitly states that ultrasound measurement of CRL in early pregnancy provides more accurate estimation of gestational age than LMP dating, with accuracy to within 7 days. 1, 2
Critical Action Steps
Reinterpret all maternal serum screening results using the corrected ultrasound-based gestational age, as screening performance is highly dependent on precise dating. 1
Obtain a new blood sample if the original sample was drawn at less than 15 weeks gestation with the corrected gestational age documented. 1
Do not average the LMP and CRL dates - always use the CRL-based date when available in the first trimester. 2, 3
Why This Matters Clinically
Maternal serum AFP levels increase by approximately 10-15% per week in the second trimester, making even small dating errors clinically significant for screening interpretation. 1, 3, 4
A 2-week dating error can change a screen-negative result to screen-positive or vice versa, potentially leading to unnecessary invasive procedures or missed diagnoses. 1
The American College of Medical Genetics recommends that composite ultrasound dating is preferable to LMP dating for improving both sensitivity and specificity of screening. 1
Documentation Requirements
Record both the LMP-based and CRL-based dates in the medical record, but use the CRL-based date for all clinical decisions. 2, 3
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, 3
Document the rationale for adopting the ultrasound-based dating in the patient's chart. 1
Common Pitfalls to Avoid
Never use the LMP date when a reliable first-trimester CRL measurement is available, even if the discrepancy seems large. 2, 3
Do not reclassify screen-positive results to screen-negative based on dating changes unless the revised gestational age differs by at least 10 days, to prevent inappropriate reclassification while allowing correction of significant dating errors. 4
Ensure proper CRL measurement technique was used (greatest length from crown to rump, excluding limbs and yolk sac) before accepting the ultrasound date. 2, 3
Special Considerations for Second Trimester Dating
If ultrasound dating is performed at 14 weeks gestation or later when LMP is uncertain, biparietal diameter (BPD) measurement alone is recommended as it rules out anencephaly and improves detection of open spina bifida. 1
In the second trimester, composite ultrasound dating based on multiple measurements is accurate to within 10 days, significantly less precise than first-trimester CRL. 1, 2