Chorionic villus sampling and amniocentesis are used to detect abnormalities in a fetus. During both procedures, ultrasonography is used for guidance. In chorionic villus sampling, a sample of chorionic villi (part of the placenta) is removed by one of two methods. In the transcervical method, a doctor inserts a thin, flexible tube (catheter) through the vagina and cervix into the placenta. In the transabdominal method, a doctor inserts a needle through the abdominal wall into the placenta. In both methods, a sample of the placenta is suctioned out with a syringe and analyzed. In amniocentesis, a doctor inserts a needle through the abdominal wall into the amniotic fluid. A sample of fluid is withdrawn for analysis.