During a routine antenatal care visit in the second trimester, a pregnant woman is counselled about the recommended investigations. According to the standard antenatal care package in India, what is the most common indication for performing an ultrasound scan in the second trimester?

## Ultrasound in Antenatal Care: Second Trimester Role ### Standard ANC Package Investigations The Indian antenatal care package recommends **at least two ultrasound scans**: one in the first trimester (11–14 weeks) and one in the second trimester (18–22 weeks). The second-trimester scan is the primary opportunity for **anatomical survey** and detection of structural anomalies. ### Purpose of Second-Trimester Ultrasound **Key Point:** The second-trimester ultrasound (18–22 weeks) is primarily designed for **detailed anatomical assessment** to detect fetal anomalies and confirm fetal viability, growth, and dating. | Trimester | Timing | Primary Purpose | Key Measurements | |-----------|--------|-----------------|------------------| | First | 11–14 weeks | Dating, nuchal translucency, viability | CRL, NT | | Second | 18–22 weeks | Anatomical survey, anomaly detection | Biometry, organ systems | | Third | 28–32 weeks | Growth assessment, placental position | AC, FL, AFI, Doppler | ### Anatomical Structures Assessed in Second Trimester 1. **Head & Neck:** Cerebellum, lateral ventricles, midline echo, face, cleft palate 2. **Thorax:** Heart (4-chamber view, outflow tracts), lungs 3. **Abdomen:** Stomach, kidneys, bladder, bowel, liver 4. **Spine:** Vertebral bodies, spinal canal 5. **Limbs:** Long bones, hands, feet 6. **Placenta & Cord:** Position, number of vessels **High-Yield:** Detection of **major fetal anomalies** (neural tube defects, cardiac defects, renal agenesis, skeletal dysplasias) is the PRIMARY goal of the second-trimester scan. This allows for **informed counselling, referral for specialized care, or termination** if desired and legal. ### Why Other Options Are Secondary - **Gestational diabetes screening:** Performed via **glucose challenge test (GCT) at 24–28 weeks**, not ultrasound - **Placental position & Doppler:** Assessed in the **third trimester (28–32 weeks)** when placental position is finalized and Doppler is most predictive - **Polyhydramnios/oligohydramnios:** While AFI is measured, this is not the PRIMARY indication; assessment of amniotic fluid is secondary to anatomical survey **Clinical Pearl:** A detailed second-trimester anomaly scan can detect ~80–90% of major structural anomalies. Soft markers (echogenic bowel, choroid plexus cysts, pyelectasis) may prompt further investigation but are often benign.