A 28-year-old primigravida at 16 weeks of gestation presents for routine antenatal screening. Maternal serum alpha-fetoprotein (AFP) is markedly elevated at 5.2 MoM (multiples of median), and ultrasound shows a lemon sign and banana sign. What is the most appropriate next step in management?

Explanation

## Clinical Context The combination of elevated maternal serum AFP, lemon sign (frontal bone concavity), and banana sign (curved cerebellum) are pathognomonic for **open spina bifida** (neural tube defect). This patient requires confirmatory imaging before any further management. ## Next Step in Management **Key Point:** High-resolution ultrasound with detailed fetal anatomy assessment is the diagnostic gold standard and must precede all other interventions. Measurement of ventricular diameter helps assess for associated hydrocephalus, which determines prognosis and counselling. **High-Yield:** The "lemon-banana" sign duo has ~99% specificity for open spina bifida when both are present. Confirmation of the defect level, size, and associated anomalies (Chiari malformation, hydrocephalus) is essential before parental counselling. **Clinical Pearl:** Open spina bifida is associated with: - Elevated AFP (>2.5 MoM in ~80% of cases) - Lemon sign: frontal bone scalloping due to increased intracranial pressure - Banana sign: cerebellum compressed into posterior fossa, appearing curved - Ventriculomegaly (often present) ## Why This Approach 1. **Diagnostic confirmation** — ultrasound is non-invasive and provides anatomical detail 2. **Prognostic assessment** — ventricular diameter and defect location predict neurological outcome 3. **Parental counselling** — accurate imaging findings enable informed decision-making 4. **Timing** — at 16 weeks, there is still time for detailed assessment and discussion before viability threshold ## Management Algorithm After Diagnosis ```mermaid flowchart TD A[Elevated AFP + Lemon/Banana sign]:::outcome --> B[High-res ultrasound with detailed anatomy]:::action B --> C{Confirm open spina bifida?}:::decision C -->|Yes| D[Measure defect level, size, ventricular diameter]:::action D --> E[Detailed parental counselling]:::action E --> F{Parental choice}:::decision F -->|Continue pregnancy| G[Plan delivery at tertiary centre, neurosurgical consultation]:::action F -->|Termination| H[Provide support and referral]:::action C -->|No| I[Reassess diagnosis, exclude other causes]:::action ``` [cite:Park 26e Ch 3]