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: Gynaecology & Obstetrics
Prenatal diagnosis at 16 weeks of pregnancy can be performed using all of the following, except: 
Select and explain. 
A. Amniotic fluid, B. Maternal blood, C. Chorionic villi, D. Fetal blood
D: Fetal blood
Answer: D: Fetal blood 
Explanation: All options given in the question are correct as all of them are used for prenatal diangosis.
But, read the question very carefully. It specifically says at “16 weeks of pregnancy”.
Cordocentesis in is done at 18–20 weeks therefore we cannot include fetal blood in this answer.