What are the implications of a nuchal translucency (NT) scan at 12 weeks + 3 days indicating a gestational age of 13 weeks and 5 days?

Implications of NT Scan Showing Larger Fetal Size Than Expected

A nuchal translucency (NT) scan at 12 weeks + 3 days showing measurements consistent with a 13 weeks + 5 days fetus primarily indicates a dating discrepancy that should prompt reassessment of gestational age using the crown-rump length (CRL) measurements rather than last menstrual period.

Understanding the Dating Discrepancy

When NT scan measurements indicate a fetus larger than expected based on menstrual dating, this typically represents one of several possibilities:

• Incorrect menstrual dating: The most common explanation is that conception occurred earlier than calculated from the last menstrual period 1
• Normal biological variation: Some fetuses are naturally larger than others at the same gestational age
• Multiple gestation: Though not indicated in this case, twins can sometimes cause measurement discrepancies 1

Clinical Significance and Management

Dating Adjustment

• CRL measurements at 11-13 weeks provide an accurate estimate of gestational age within 7 days 1
• The dating should be adjusted based on the CRL measurement, as this is more reliable than menstrual dating 1, 2
• Express the new gestational age as 13 weeks + 5 days rather than rounding to completed weeks 1

NT Assessment Implications

• NT screening performance is optimal at 11 completed weeks and decreases by 13 completed weeks 1
• The NT measurement should still be valid if performed within the 11-13+6 weeks window 1
• The measurement should be interpreted using the appropriate reference range for the new gestational age 1

Further Evaluation

• If NT measurement is ≥3mm or ≥99th percentile, genetic counseling and diagnostic testing should be offered promptly 1
• With normal NT measurements, the risk of adverse outcomes is significantly reduced 3
• A detailed anatomy scan at 18-22 weeks should still be performed as scheduled 1

Important Considerations

• First-trimester ultrasound dating is highly reliable and should take precedence over menstrual dating when discrepancies exist 1, 2
• Early dating is crucial for accurate interpretation of subsequent screening tests and management of the pregnancy 1
• The detection rate for major structural anomalies at 12-13 weeks is approximately 45-50%, with particularly severe anomalies being more likely to be detected early 4

Common Pitfalls to Avoid

• Don't retain the original due date: When CRL measurements differ from menstrual dating by more than 7 days in the first trimester, the due date should be adjusted based on the ultrasound 1
• Don't miss the opportunity for NT screening: Even with the dating adjustment, NT screening is still valid if performed within the 11-13+6 weeks window 1
• Don't overinterpret size discrepancy alone: Without other concerning findings, a larger-than-expected CRL is most likely due to dating error rather than pathology 1, 2

In summary, a fetus measuring larger than expected at the NT scan primarily indicates a need to adjust dating rather than a concerning finding, assuming the NT measurement itself is normal.