Amniocentesis and chorionic villus sampling.
Amniocentesis performed at 16 to 18 weeks' gestation has been the gold standard approach for prenatal cytogenetic diagnosis. Over the past few years, large collaborative studies on chorionic villus sampling have confirmed the safety and efficacy of chorionic villus sampling as a viable alternative for women seeking prenatal diagnosis. While the expanding experience with chorionic villus sampling has answered questions regarding the safety of the transabdominal approach, it has also raised questions concerning possible associations with limb abnormalities, its usefulness in multiple gestations, and the clinical significance of confined placental mosaicism. These issues, as well as the technical and gestational age limitations of chorionic villus sampling, have led many investigators to study the technical feasibility, safety, and accuracy of amniocentesis performed in the first trimester or early second trimester. While this approach appears both safe and efficacious, there are concerns regarding orthopedic abnormalities and the reliability of first-trimester amniotic fluid acetylcholinesterase and alpha-fetoprotein levels in the diagnosis of neural tube defects.