This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Obstetrics, Subject name: Gynaecology & Obstetrics
Karyotyping of the fetus can be done routinely through all of the following invasive method except: 
Select and explain. 
A. Chorionic villus sampling, B. Cordocentesis, C. Amniocentesis, D. Fetal skin biopsy
D: Fetal skin biopsy
Answer: D: Fetal skin biopsy 
Explanation: There are three routine ways of obtaining fetal tissue for karyotyping: Chorionic villus sampling - done at First trimester(10-13th weeks) Amniocentesis - done at second trimester(16-18th weeks) Percutaneous umbilical blood sampling (cordocentesis) - done after 18 weeks.