This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Medical & Surgical Illness Complication Pregnancy, Subject name: Gynaecology & Obstetrics
A woman develops chickenpox at a period of gestation of 8 weeks.What is the best method to exclude foetal varicella syndrome? 
Select and explain. 
A. Perform amniocentesis at 15 weeks and test for VZV DNA in the amniotic fluid, B. Perform an ultrasound scan at 20 weeks, C. Perform an ultrasound scan immediately, D. Perform chorionic villous sampling and test for VZV DNA
B: Perform an ultrasound scan at 20 weeks
Answer: B: Perform an ultrasound scan at 20 weeks 
Explanation: Ans. is b, i.e. Perform an ultrasound scan at 20 weeks.A detailed ultrasound examination carried out 5 weeks after infection, or at 16-20 weeks is the best method to exclude fetal varicella syndrome.Amniocentesis is not routinely advised because the risk of congenital varicella syndrome is low, even when amniotic fluid is positive for VZV DNA because amniocentesis has a strong negative productive value than a positive predictive valve.