A 28-year-old primigravida at 16 weeks of gestation attends the antenatal clinic for routine screening. She is asymptomatic with normal vital signs. As per the National Guidelines for Antenatal Care in India, which investigation is the most appropriate choice for detection of chromosomal abnormalities at this gestational age?

Explanation

## Screening for Chromosomal Abnormalities in Second Trimester **Key Point:** The quadruple marker test (also called quad screen or quadruple screening) is the investigation of choice for aneuploidy screening in the second trimester (15–20 weeks) in India's standard antenatal care package. ### Biochemical Markers Measured | Marker | Normal Finding | Abnormal Pattern (Down Syndrome) | |--------|---|---| | α-fetoprotein (AFP) | Elevated | Decreased | | Human chorionic gonadotropin (hCG) | Normal | Increased | | Unconjugated estriol (uE3) | Normal | Decreased | | Inhibin A | Normal | Increased | **High-Yield:** The quadruple test has a detection rate of ~80% for Down syndrome (Trisomy 21) and ~60% for Edwards syndrome (Trisomy 18) with a false-positive rate of 5%. ### Timing and Rationale - **16 weeks gestation:** Optimal window for quad screening is 15–20 weeks (second trimester). - **Non-invasive:** No risk of miscarriage, unlike invasive procedures. - **Cost-effective:** Affordable and widely available in India. - **Guideline-recommended:** Part of the standard antenatal care package per NRHM/NITI Aayog guidelines. **Clinical Pearl:** A positive quad screen (risk >1:250 for Down syndrome) warrants counselling and discussion of further testing options (detailed ultrasound, NIPT, or invasive testing). ### Why Quad Screening at 16 Weeks? 1. Biochemical markers are most reliable in the second trimester. 2. Allows time for decision-making before viability threshold (20 weeks). 3. Combines maternal age + biochemical markers for improved detection. **Mnemonic:** **QUAD** = **Q**uadruple markers in second trimester for **A**neuploidy **D**etection.