**Core Concept**
The ultrasound examination (USG) at 18-20 weeks gestation is a critical milestone in prenatal care, used to assess fetal development and detect potential anomalies. This period is crucial for identifying congenital abnormalities, estimating fetal weight, and monitoring fetal growth.

**Why the Correct Answer is Right**
The primary purpose of the USG at 18-20 weeks is to perform a detailed anatomy scan, also known as a level II ultrasound. This scan involves a thorough examination of the fetal anatomy, including the brain, spine, heart, abdomen, and limbs. The sonographer uses high-frequency transvaginal or transabdominal ultrasound probes to visualize the fetus in detail, allowing for the detection of potential structural anomalies and other complications. The detailed anatomy scan at 18-20 weeks is essential for identifying conditions such as neural tube defects, heart defects, and abdominal wall defects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while USG at 18-20 weeks can estimate fetal weight, it is not the primary purpose of this scan. Fetal weight estimation is a secondary benefit of the detailed anatomy scan.

**Option B:** This option is incorrect because USG at 18-20 weeks is not primarily used for fetal monitoring. Fetal monitoring is typically done through non-stress tests (NSTs) and biophysical profiles (BPPs) later in pregnancy.

**Option C:** This option is incorrect because while USG at 18-20 weeks can detect fetal anomalies, it is not primarily used for fetal growth assessment. Fetal growth assessment is typically done through serial ultrasound measurements and estimated fetal weight calculations.

**Clinical Pearl / High-Yield Fact**
A detailed anatomy scan at 18-20 weeks gestation is critical for detecting congenital anomalies, which are present in approximately 2-3% of births. The most common anomalies detected during this period include neural tube defects (e.g., spina bifida), heart defects, and abdominal wall defects.

**Correct Answer:** C.