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: Pediatrics
The maximum risk of developing foetal varicella syndrome is when the exposure is between what period of pregnancy? 
Select and explain. 
A. < 9 weeks, B. 13 -20 weeks, C. 20-24 weeks, D. 4 days before delivery
B: 13 -20 weeks
Answer: B: 13 -20 weeks 
Explanation: 13-20 weeks is the critical period for foetal varicella syndrome.