Amniocentesis and chorionic villus sampling.

Amniocentesis and chorionic villus sampling (CVS) remain the most commonly used invasive prenatal diagnostic procedures. Recent reports on early amniocentesis demonstrate its application to the prenatal detection of certain biochemical disorders. However, its role in the evaluation of open fetal defects of the neural tube or ventral wall is still under investigation. The fact that many reports concerning early amniocentesis include a majority of patients beyond 11 to 12 weeks' gestation, thus placing the procedure outside the first trimester, make comparisons with CVS (usually performed between 9 and 11 weeks) problematic. The role of midtrimester amniocentesis in evaluating elevations of maternal serum alpha-fetoprotein, following a normal ultrasonographic examination performed specifically to detect fetal anomalies, is under scrutiny. It appears that risk adjustment may be appropriate following a normal scan, and prior to invasive procedures, but each center's recommendation to a given patient will depend on the expertise of the individual sonographer, as well as the quality of examination. CVS has gained acceptance as a safe first-trimester means of prenatal diagnosis, with increasing applications in the later stages of pregnancy. Chromosomal mosaicism detected by CVS may represent a phenomenon inherent to placental tissue; questions remain regarding mosaicism as a potential marker for increased pregnancy loss. Comparisons between the transcervical and transabdominal routes are reviewed, with equivalent results regarding safety and efficacy. Recent evaluations of fetomaternal transfusion following CVS are also described.