Interpret NT measurement for chromosomal screening.
NT is measured at 11–14 weeks (CRL 45–84 mm). Normal NT increases with CRL; values ≥ 3.0 mm are elevated regardless of CRL. NT is part of combined first-trimester screening (with PAPP-A and free beta-hCG). Elevated NT increases risk for trisomy 21, 18, 13, and cardiac defects. NT alone is not diagnostic – it adjusts baseline risk.