A 28-year-old primigravida attends her first antenatal clinic visit at 12 weeks of gestation. During the booking visit, routine investigations are ordered. Which is the most common abnormality detected on the first-trimester combined screening (nuchal translucency + biochemical markers) in Indian pregnancies?

Explanation

## Most Common Chromosomal Abnormality in First-Trimester Screening **Key Point:** Down syndrome (Trisomy 21) is the most frequently detected chromosomal abnormality on first-trimester combined screening across all populations, including India. ### Epidemiology of Chromosomal Abnormalities | Condition | Incidence (live births) | Detection Rate (1st trimester) | Maternal Age Association | |-----------|-------------------------|--------------------------------|---------------------------| | Down syndrome (T21) | 1 in 700–1000 | 85–90% | Strong (increases >35 years) | | Edwards syndrome (T18) | 1 in 6000 | 80–85% | Strong | | Patau syndrome (T13) | 1 in 10,000 | 75–80% | Moderate | | Turner syndrome (45,X) | 1 in 2000–2500 | 60–70% | Weak | **High-Yield:** Down syndrome is the most common viable autosomal trisomy. Although many T18 and T13 conceptions occur, they have much higher intrauterine lethality rates, making T21 the most frequent live birth abnormality and the most commonly detected on screening. ### First-Trimester Combined Screening Components (11–14 weeks) 1. **Nuchal translucency (NT)** — ultrasound measurement of fluid at the nape of the fetal neck 2. **PAPP-A** (pregnancy-associated plasma protein A) — typically decreased in T21 3. **β-hCG** (beta human chorionic gonadotropin) — typically elevated in T21 **Clinical Pearl:** The combination of increased NT + elevated β-hCG + decreased PAPP-A has the highest sensitivity for Down syndrome detection (~85–90%). **Mnemonic:** **PAPP-A DOWN** — PAPP-A is Decreased in Down syndrome; β-hCG is elevated (UP). ### Why Down Syndrome is Most Common - **Highest live birth incidence** among chromosomal abnormalities (1 in 700–1000 in India, varies by maternal age) - **Viable** — most T21 conceptions survive to term (unlike T18/T13 with ~90% fetal loss) - **Maternal age effect** — risk increases exponentially after age 35, and advanced maternal age is common in many Indian populations - **Screening sensitivity** — first-trimester combined screening detects ~85–90% of T21 cases, making it the most reliably identified abnormality **Warning:** Do not confuse incidence with detection rate. Although T18 and T13 have high detection rates (80–85%), their overall incidence is much lower, so T21 remains the most commonly detected abnormality in absolute numbers. [cite:RCOG Green-top Guideline 67 on Antenatal Screening]