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
[{'content': 'Karyotyping of the fetus can be done routinely through all of the following invasive method except: \nSelect and explain. \nA. Chorionic villus sampling, B. Cordocentesis, C. Amniocentesis, D. Fetal skin biopsy', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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.', 'role': 'assistant'}]