Prenatal diagnosis in twin gestations: A comparison between second-trimester amniocentesis and first-trimester chorionic villus sampling

Objective: To evaluate prospectively the relative risks and accuracy of first-trimester chorionic villus sampling (CVS) and second-trimester amniocentesis in the genetic evaluation of twin gestations. Methods: Between March 1984 and August 1990, patients presenting for prenatal diagnosis of a twin gestation of less than 12 weeks were offered sampling by either first-trimester CVS or amniocentesis at 16-18 weeks' gestation. Selection was based solely on patient preference and was obtained before ultrasound identification of placental position. Women presenting beyond 12 weeks' gestation were sampled by amniocentesis. Clinical and laboratory outcomes were evaluated. Results: Eighty-one women had amniocentesis (nine of whom also had CVS), and 161 women had CVS