Invasive methods of prenatal diagnosis: In Chorion villus sampling, Amniocentesis and Cordocentesis fetal cells are obtained for karyotyping.Fetoscopy is done at 16-20 weeks, for visualization of fetal external anatomy and for fetal skin or muscle biopsy for diagnosis of genetic disorders (Meckel-Gruber syndrome). Reference:Dutta,s Textbook of obstetrics 7th edition page no 494