On first-trimester ultrasound at 13 weeks, a fetal nuchal translucency (NT) of 4.5 mm is measured. What is the most appropriate next step?
- A Chorionic villus sampling (CVS) or cell-free fetal DNA with detailed fetal echocardiography ✓
- B Repeat NT at 16 weeks
- C Amniocentesis at 16 weeks for karyotype
- D Routine follow-up; NT <3 mm is the only concerning threshold
Correct answer: A. Chorionic villus sampling (CVS) or cell-free fetal DNA with detailed fetal echocardiography
Explanation
Nuchal translucency >3.5 mm (or >2.5 MoM) is significantly elevated and carries a high risk of chromosomal anomalies (trisomy 21, 18, 13, Turner syndrome) and structural defects (cardiac). CVS (for early karyotype/chromosomal microarray) and cell-free fetal DNA are offered. Detailed fetal echocardiography is warranted given the strong association with congenital heart disease. Amniocentesis at 16 weeks is an option but CVS is preferred for earlier diagnosis. Repeat NT has no diagnostic value.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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