During routine antenatal care at 16 weeks of gestation, a 32-year-old woman undergoes the triple screen (maternal serum screening). Which is the most common chromosomal abnormality detected by this test in the Indian population?

Explanation

## Maternal Serum Screening (Triple Screen) and Chromosomal Abnormalities ### Overview of Triple Screen **Key Point:** The triple screen (performed at 15–20 weeks) measures three maternal serum markers: - Alpha-fetoprotein (AFP) - Human chorionic gonadotropin (hCG) - Unconjugated estriol (uE3) ### Marker Pattern in Trisomy 21 | Marker | Pattern in Down Syndrome | Mechanism | |--------|--------------------------|----------| | **AFP** | ↓ Decreased | Reduced fetal production | | **hCG** | ↑ Increased | Elevated placental secretion | | **uE3** | ↓ Decreased | Reduced fetal adrenal synthesis | | **Inhibin A** | ↑ Increased | (Quadruple screen adds this) | ### Prevalence of Chromosomal Abnormalities | Abnormality | Prevalence at Birth | Detection Rate by Triple Screen | |-------------|-------------------|--------------------------------| | **Trisomy 21** | 1 in 700–1000 | 60–70% (75–85% with quad screen) | | Trisomy 18 | 1 in 6000 | 60% | | Trisomy 13 | 1 in 10,000 | 50% | | Turner syndrome | 1 in 2000–2500 | <10% (not reliably detected) | ### High-Yield Facts **High-Yield:** Trisomy 21 is the most common viable chromosomal abnormality at birth and the most frequently detected by maternal serum screening. It accounts for ~50% of all chromosomal abnormalities in the antenatal period. **Key Point:** The triple screen has a detection rate of 60–70% for Down syndrome at a 5% false-positive rate. The quadruple screen (adding inhibin A) improves detection to 80–85%. ### Clinical Pearl **Clinical Pearl:** In the Indian population, advanced maternal age (>35 years) and consanguinity increase the risk of trisomy 21. Screening should be offered to all pregnant women regardless of age, as ~80% of Down syndrome cases occur in women <35 years. ### Management Algorithm ```mermaid flowchart TD A[Maternal serum screening at 15-20 weeks]:::action --> B{Risk assessment}:::decision B -->|Low risk| C[Routine follow-up]:::outcome B -->|Intermediate risk| D[Offer detailed ultrasound + counselling]:::action B -->|High risk| E[Offer invasive testing: CVS or amniocentesis]:::action E --> F{Invasive test result}:::decision F -->|Abnormal| G[Genetic counselling, delivery planning]:::outcome F -->|Normal| H[Reassurance, routine care]:::outcome ``` **Mnemonic: Down 21** — **D**ecreased AFP, **D**ecreased uE3, **U**p hCG, **N**eed further testing.