A 28-year-old primigravida presents at 16 weeks of gestation for her second antenatal visit. She is asymptomatic with a blood pressure of 120/78 mmHg and weight gain of 2 kg since booking. Urine dipstick shows trace protein. Hemoglobin is 11.2 g/dL. She asks which screening test should be performed at this visit. What is the most appropriate investigation at this gestational age?

## Timing of Antenatal Screening Tests **Key Point:** The second trimester quad screen is the gold standard biochemical screening test for chromosomal abnormalities and should be performed between 15–20 weeks of gestation, ideally at 16–18 weeks. ### Quad Screen Components The quad screen measures four maternal serum markers: - **AFP (alpha-fetoprotein)** — decreased in Down syndrome (trisomy 21) - **hCG (human chorionic gonadotropin)** — increased in Down syndrome - **uE3 (unconjugated estriol)** — decreased in Down syndrome - **Inhibin A** — increased in Down syndrome **High-Yield:** The quad screen has a detection rate of ~80% for trisomy 21 and ~75% for trisomy 18 with a false-positive rate of ~5%. ### Comparison of Screening Tests by Gestational Age | Test | Gestational Age | Detection Rate | Notes | |------|-----------------|----------------|-------| | Nuchal translucency (NT) | 11–14 weeks | 75–80% | First trimester screening | | Quad screen | 15–20 weeks | ~80% for T21 | Second trimester; biochemical | | Glucose tolerance test | 24–28 weeks | — | Gestational diabetes screening | | Detailed anatomy scan | 18–22 weeks | — | Structural anomalies; done after quad screen | **Clinical Pearl:** At 16 weeks, the patient is in the optimal window for quad screen. The nuchal translucency scan is too late (should be done by 14 weeks), and the glucose tolerance test and detailed anatomy scan are performed later. ### Antenatal Visit Schedule in India (NRHM Guidelines) 1. **First visit (≤12 weeks):** Dating scan, baseline investigations, blood pressure, weight 2. **Second visit (16–18 weeks):** Quad screen, repeat BP, weight, urine 3. **Third visit (18–22 weeks):** Detailed anatomy scan (fetal anomaly scan) 4. **Subsequent visits:** Monthly until 28 weeks, then fortnightly until 36 weeks, then weekly **Warning:** Do not confuse the timing of quad screen (15–20 weeks) with nuchal translucency (11–14 weeks) or detailed anatomy scan (18–22 weeks).