**Core Concept**
Fetal anatomy scan, also known as level II ultrasound, is a comprehensive assessment of fetal growth, structure, and function. This examination is crucial for identifying potential congenital anomalies and monitoring fetal well-being during the second trimester.

**Why the Correct Answer is Right**
The primary purpose of an antenatal ultrasound at 18-20 weeks of gestation is to detect congenital anomalies, such as neural tube defects (e.g., spina bifida), cardiac malformations, and limb abnormalities. The ultrasound provides a detailed assessment of fetal anatomy, allowing for the identification of potential issues. The sonographer evaluates the fetal profile, skull, spine, limbs, and major organs, including the heart, lungs, liver, and kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is vague and does not specify a particular condition or purpose. While an ultrasound may detect various fetal abnormalities, this option does not provide a clear answer.
**Option B:** Fetal viability is typically assessed in the second trimester, but it is not the primary purpose of an ultrasound at 18-20 weeks.
**Option C:** Fetal weight estimation may be performed during the ultrasound, but it is not the primary purpose of this examination.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women undergo a fetal anatomy scan between 16-22 weeks of gestation to detect potential congenital anomalies.

**Correct Answer:** C. Detection of congenital anomalies.