A 28-year-old primigravida presents at 16 weeks gestation for her second antenatal visit. She is asymptomatic with no significant past medical history. On examination, fundal height is 14 cm and vital signs are normal. She had a normal first trimester scan at 12 weeks. The obstetrician orders a second trimester anomaly scan and maternal serum screening (quadruple marker test). The quadruple marker results show: AFP 0.8 MoM, hCG 2.1 MoM, uE3 0.9 MoM, and inhibin A 2.3 MoM. What is the most appropriate next step in management?

Explanation

## Clinical Context This patient has a **combined screening result suggestive of increased risk for Down syndrome (Trisomy 21)**. The pattern of elevated hCG and inhibin A with normal AFP and uE3 is characteristic of Down syndrome. ## Interpretation of Quadruple Marker Results **Key Point:** The quadruple marker test (also called the "quad screen" or "quad test") measures four serum analytes in the second trimester (15–22 weeks, optimal 16–18 weeks): - **AFP (α-fetoprotein):** Decreased in Down syndrome (0.8 MoM is low-normal) - **hCG (human chorionic gonadotropin):** Elevated in Down syndrome (2.1 MoM is elevated) - **uE3 (unconjugated estriol):** Decreased in Down syndrome (0.9 MoM is low-normal) - **Inhibin A:** Elevated in Down syndrome (2.3 MoM is elevated) **High-Yield:** A **risk cutoff of 1 in 250** at mid-trimester is typically used. When combined risk exceeds this threshold, invasive testing should be offered. ## Management Algorithm ```mermaid flowchart TD A[Quadruple marker screening at 15-22 weeks]:::outcome --> B{Risk calculation}:::decision B -->|Risk ≥ 1:250| C[Detailed ultrasound + genetic counseling]:::action B -->|Risk < 1:250| D[Routine antenatal care]:::action C --> E{Ultrasound findings abnormal?}:::decision E -->|Yes| F[Offer amniocentesis/CVS]:::action E -->|No| G[Reassess risk; discuss options]:::action G --> H{Patient chooses invasive test?}:::decision H -->|Yes| I[Amniocentesis or CVS]:::action H -->|No| J[Continue surveillance]:::action ``` ## Correct Management Approach 1. **Detailed ultrasound:** Look for soft markers (nuchal fold, cardiac defects, renal pyelectasis, absent nasal bone, shortened femur/humerus) that refine risk. 2. **Risk calculation:** Integrate serum markers with ultrasound findings and maternal age to generate **individualized risk**. 3. **Counseling:** Discuss the increased risk, explain what amniocentesis involves (procedure risk ~0.1–0.3%), and allow informed choice. 4. **Invasive testing if indicated:** Amniocentesis or chorionic villus sampling (CVS) for definitive karyotype. **Clinical Pearl:** Modern practice integrates first-trimester combined screening (nuchal translucency + PAPP-A + hCG) with second-trimester markers or uses **non-invasive prenatal testing (NIPT)** for higher detection and lower false-positive rate. However, in this scenario, the quad screen result must be acted upon by offering detailed assessment and counseling. **Warning:** Simply repeating the test or reassuring without risk stratification is inappropriate — elevated hCG and inhibin A with this pattern warrant further evaluation. [cite:Williams Obstetrics 26e Ch 10]