A 24-year-old primigravida from Delhi presents for her first antenatal scan at 12 weeks of gestation (by last menstrual period). Transvaginal ultrasound shows a crown-rump length (CRL) of 48 mm. The sonographer notes increased nuchal translucency (NT) measuring 3.2 mm. There is no fetal bradycardia, and the nasal bone is visualized. What is the most appropriate next step in management?

Explanation

## Interpretation of First-Trimester Markers **Key Point:** Increased nuchal translucency (NT) at 11–14 weeks is a soft marker for chromosomal abnormalities, particularly trisomy 21, but is NOT diagnostic on its own. ### NT Measurement and Risk Stratification | Finding | Significance | |---------|-------------| | NT 2.5–3.5 mm at 12 weeks | Borderline; requires further risk assessment | | NT > 3.5 mm | Increased risk; warrants invasive testing or cfDNA | | Presence of nasal bone | Reassuring; reduces trisomy 21 risk by ~50% | | Absence of nasal bone | Increases trisomy 21 risk | | Normal heart rate | Reassuring; absence of bradycardia reduces risk | **Clinical Pearl:** The combination of borderline NT (3.2 mm) + visualized nasal bone + normal heart rate creates a mixed risk profile. This is NOT a high-risk finding requiring immediate invasive testing, but it warrants further stratification. ### Management Algorithm ```mermaid flowchart TD A[First-trimester scan: 11-14 weeks]:::outcome --> B{NT measurement}:::decision B -->|NT < 2.5 mm| C[Low risk; routine care]:::action B -->|NT 2.5-3.5 mm| D{Nasal bone present?}:::decision B -->|NT > 3.5 mm| E[High risk]:::urgent D -->|Yes| F[Intermediate risk; offer cfDNA or invasive testing]:::action D -->|No| E E --> G[Offer cfDNA or invasive testing urgently]:::action F --> H[Patient counseling & choice]:::action G --> H ``` **High-Yield:** First-trimester combined screening (NT + nasal bone + maternal age + serum markers) has ~90% detection rate for trisomy 21 with 5% false-positive rate. cfDNA testing (non-invasive prenatal testing, NIPT) is now the preferred second-line test in most centers. **Mnemonic: FIRST-TRIMESTER MARKERS** — Nasal bone (N), Tricuspid regurgitation (T), Ductus venosus (D), Nuchal translucency (NT), and maternal age (MA). ## Why This Answer Is Correct The patient has a borderline NT with reassuring features (nasal bone present, normal heart rate). This warrants risk stratification via cfDNA testing or invasive testing after appropriate counseling—not immediate reassurance, not immediate invasive testing, and certainly not pregnancy termination without definitive diagnosis.