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What is Karyotyping of a fetus is done routinely by all the following except and explain in detail?
Karyotyping: the Visual study of chromosomes after metaphase arrest by the use of ColchicineFetal karyotyping :1)Chorionic villus sampling: Biopsy taken transcervically at 8 to 12 weeks of gestation2)Amniocentesis: 20 ml of amniotic fluid is aspirated at 14 to 20 weeks of gestation3)Cordocentesis (not preferable): Fetal blood aspirated from the umbilical cord after 18weeks of gestation(Ref: Kawthalkar's essentials of hematology 2/e p162)