## Screening for Chromosomal Abnormalities in Second Trimester ### Optimal Timing and Test Choice **Key Point:** The triple marker test (or quadruple marker test) is the investigation of choice for chromosomal abnormality screening in the second trimester (15–20 weeks) in India, as per the standard antenatal care package. ### Comparison of Investigations | Investigation | Timing | Type | Detection Rate | Risk | Use in India | |---|---|---|---|---|---| | **Triple/Quadruple Marker** | 15–20 weeks | Screening | 60–80% (Down syndrome) | Non-invasive | Standard ANC package | | CVS | 11–13 weeks | Diagnostic | >99% | 0.2–0.5% miscarriage | Invasive; for high-risk cases | | NIPT | 10+ weeks | Screening | 99% | Non-invasive | Emerging; not routine in public sector | | Amniocentesis | 15–20 weeks | Diagnostic | >99% | 0.1–0.3% miscarriage | Invasive; confirmatory only | ### Why Triple/Quadruple Marker at 16 Weeks? 1. **Timing:** Falls within the optimal window (15–20 weeks) for accurate marker detection. 2. **Non-invasive:** No risk of miscarriage; suitable for population screening. 3. **Cost-effective:** Affordable and widely available in India. 4. **Integrated approach:** Often combined with first-trimester nuchal translucency (NT) ultrasound (if available) for improved detection. **High-Yield:** The quadruple marker test (hCG, uE3, AFP, inhibin A) is more sensitive than the triple marker for Down syndrome (trisomy 21) detection. ### Clinical Pearl **Key Point:** If the triple/quadruple marker is abnormal or high-risk, then invasive testing (amniocentesis or CVS) is offered for definitive diagnosis, not as a first-line screening tool. ### Mnemonic: **TQMA** - **T**riple/Quadruple marker → **Screening** (2nd trimester, non-invasive) - **Q**uadruple (hCG, uE3, AFP, inhibin A) → Better than triple - **M**arkers → Maternal serum biochemistry - **A**mniocentesis → **Diagnostic** (only if screening is abnormal)
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