A 28-year-old primigravida at 16 weeks gestation presents for routine antenatal care. Serum alpha-fetoprotein (AFP) is elevated at 2.5 MoM (multiples of median). She is anxious about fetal anomalies. What is the most appropriate next investigation?

Explanation

## Investigation Strategy for Elevated AFP in Second Trimester **Key Point:** Elevated AFP (>2.0 MoM) at 15–20 weeks warrants detailed anatomical ultrasound as the first-line confirmatory investigation, not invasive testing. ### Rationale for High-Resolution Ultrasound Elevated AFP is associated with: - Open neural tube defects (spina bifida, anencephaly) - Abdominal wall defects (gastroschisis, omphalocele) - Chromosomal abnormalities (Down syndrome, Edwards syndrome) - Placental abnormalities Ultrasound can directly visualize most structural anomalies and refine risk before considering invasive testing. ### Why This Approach? 1. **Non-invasive** — no risk of miscarriage 2. **High sensitivity** for neural tube defects (>95%) and abdominal wall defects 3. **Diagnostic** — often resolves the cause (e.g., open spina bifida is visible) 4. **Guides next steps** — if anatomy is normal, reassurance; if abnormality found, counseling and invasive testing offered ### Timing Optimal timing is **18–20 weeks** for detailed anatomy scan, though 16 weeks is acceptable if high-quality equipment and expertise available. **Clinical Pearl:** Elevated AFP + normal detailed ultrasound = reassuring; risk of chromosomal abnormality is reduced but not eliminated. Offer maternal serum cell-free DNA or amniocentesis if patient remains at high risk or if ultrasound findings are equivocal. **High-Yield:** The "triple screen" (AFP, hCG, uE3) or "quad screen" (adds inhibin A) are screening tests; ultrasound is the confirmatory investigation for abnormal biochemistry. [cite:Park 26e Ch 6]