A 28-year-old primigravida at 16 weeks of gestation attends her first antenatal visit. She is asymptomatic with a normal obstetric examination. Her booking blood tests show: Hb 11.2 g/dL, blood group O positive, negative infectious serology (HIV, HBsAg, VDRL, rubella IgM). She has no significant past medical history. What is the most appropriate next investigation at this visit?

Explanation

## Investigation Timing and Appropriateness in Early Pregnancy ### Gestational Age Context At 16 weeks of gestation, the patient is in the second trimester. This is the optimal window for second-trimester biochemical screening. ### Quadruple Marker Screening **Key Point:** Quadruple marker screening (also called quad screen or quad test) is performed between 15 and 22 weeks of gestation, with optimal timing at 16–18 weeks. **High-Yield:** The quad screen measures four maternal serum markers: - Alpha-fetoprotein (AFP) — decreased in Down syndrome - Human chorionic gonadotropin (hCG) — elevated in Down syndrome - Unconjugated estriol (uE3) — decreased in Down syndrome - Inhibin A — elevated in Down syndrome This combination detects approximately 80–85% of Down syndrome cases and 70% of Edwards syndrome (Trisomy 18) cases with a false-positive rate of ~5%. ### Why Quad Screen at 16 Weeks? **Clinical Pearl:** The quad screen is the standard first-tier screening test for chromosomal abnormalities in the second trimester in India and most resource-limited settings. It is cost-effective, non-invasive, and has good detection rates. At 16 weeks, the patient is at the ideal gestational age window. ### Comparison with Other Options | Investigation | Timing | Purpose | Indication | |---|---|---|---| | Nuchal translucency (NT) | 11–14 weeks | First-trimester screening | First trimester; too late at 16 weeks | | Quad screen | 15–22 weeks (optimal 16–18) | Second-trimester screening | Standard at 16 weeks | | NIPT (cfDNA) | 10+ weeks | Non-invasive prenatal testing | High-risk pregnancies; not standard first-line in resource-limited settings | | Detailed morphology scan | 18–22 weeks | Anatomical assessment | Later in second trimester; done after biochemical screening | **Mnemonic:** **QUAD** = **Q**uick **U**sefulness **A**t **D**ays 105–154 (15–22 weeks). ### Rationale for Quad Screen Here 1. **Timing:** 16 weeks is within the optimal window. 2. **Cost-effectiveness:** Quad screen is affordable and widely available in India. 3. **Detection rate:** 80–85% for Down syndrome. 4. **Non-invasive:** No risk of miscarriage, unlike invasive testing. 5. **Standard of care:** Recommended by RCOG, ACOG, and Indian guidelines for antenatal screening.