USG done at 18 – 20 weeks mainly to :
**Core Concept**
Fetal anatomy scanning is crucial during the second trimester, particularly between 18-20 weeks of gestation, to assess fetal development and detect potential congenital anomalies. This period offers a unique window for detailed ultrasound evaluation due to the fetus's size and position.
**Why the Correct Answer is Right**
The primary goal of ultrasound scanning at 18-20 weeks is to perform a detailed anatomical survey, focusing on major organs and body systems. This examination, also known as a Level II ultrasound, is essential for identifying potential congenital anomalies, such as cardiac defects, neurological abnormalities, and gastrointestinal or genitourinary tract malformations. The sonographer or radiologist examines the fetal anatomy in detail, including the brain, spine, heart, lungs, liver, kidneys, and limbs, to identify any deviations from normal development.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because while fetal growth assessment is an important aspect of prenatal care, it is not the primary purpose of the ultrasound at 18-20 weeks.
**Option B:** Dating the pregnancy is typically done earlier in the gestation, around 10-14 weeks, using crown-rump length measurements.
**Option C:** Estimated fetal weight is an indirect measure that is often calculated later in pregnancy, usually after 28 weeks.
**Clinical Pearl / High-Yield Fact**
The 18-20 week ultrasound is a critical milestone in prenatal care, providing valuable information about fetal development and potential congenital anomalies. This examination is often referred to as the "anatomy scan" or "Level II ultrasound."
**Correct Answer: C.**