A 24-year-old primigravida from rural Maharashtra presents to the antenatal clinic for her first ultrasound at what she reports as 20 weeks of gestation based on last menstrual period. On transabdominal ultrasound, the biparietal diameter (BPD) measures 42 mm, femur length (FL) is 28 mm, and head circumference (HC) is 156 mm. Additionally, the sonographer notes absence of the nasal bone on sagittal view. The amniotic fluid volume is normal. What is the most appropriate next step in management?

Explanation

## Clinical Context This is a case of suspected chromosomal abnormality (likely Down syndrome/Trisomy 21) based on a combination of sonographic markers in the second trimester. ## Key Ultrasound Findings | Finding | Measurement | Interpretation | |---------|-------------|----------------| | BPD | 42 mm | Consistent with ~18–19 weeks (smaller than expected for 20 weeks) | | FL | 28 mm | Consistent with ~18 weeks (short for stated 20 weeks) | | HC | 156 mm | Consistent with ~18–19 weeks | | Nasal bone | Absent | Soft marker for aneuploidy (present in ~70% of Down syndrome cases) | **Key Point:** Absent or hypoplastic nasal bone in the second trimester is a significant soft marker for trisomy 21, with a likelihood ratio of approximately 5–10 for Down syndrome. ## Appropriate Management 1. **Confirm dating:** The biometric parameters suggest the patient is actually at ~18–19 weeks, not 20 weeks. This discrepancy should be documented. 2. **Detailed survey:** Perform a comprehensive fetal anatomy scan looking for other markers: - Increased nuchal fold thickness (>6 mm at 15–20 weeks) - Cardiac defects (AV canal, VSD) - Renal pyelectasis - Echogenic bowel - Short femur or humerus 3. **Genetic counseling:** Discuss the significance of nasal bone absence and offer: - Cell-free fetal DNA testing (non-invasive prenatal testing, NIPT) — first-line in most centres - Amniocentesis for definitive karyotyping if NIPT is positive or declined 4. **Avoid premature reassurance or termination:** Neither extreme is appropriate without further investigation. **High-Yield:** Absent nasal bone + maternal age + other soft markers → offer invasive or non-invasive genetic testing. This is the standard of care in modern obstetric imaging [cite:ISUOG Guidelines on Screening for Fetal Abnormalities 2019]. **Clinical Pearl:** The combination of dating discrepancy (biometry 1–2 weeks behind) + absent nasal bone + normal amniotic fluid is a classic presentation for Down syndrome, not a measurement error or normal variant. ## Why Not the Other Options - **Option 0 (reassurance):** Absent nasal bone is NOT a normal variant; it requires investigation. - **Option 2 (immediate termination):** Premature without confirmatory genetic testing; many Down syndrome pregnancies are carried to term, and the diagnosis must be confirmed first. - **Option 3 (repeat in 2 weeks):** Delays appropriate counseling and testing; the findings are significant now and warrant action.