This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
USG done at 18 - 20 weeks mainly to : 
Select and explain. 
A. Detect fetal abnormality, B. Determine sex, C. Estimate liquor, D. Determine maturity
A: Detect fetal abnormality
Answer: A: Detect fetal abnormality 
Explanation: At 18 - 20th weeks, 2nd scan is done mainly to look for congenital anomalies.