Invasive prenatal diagnostic techniques.

As aneuploid screening evolves to the first trimester a complimentary diagnostic procedure becomes increasingly important. Chorionic villus (CVS) sampling has emerged as the only safe invasive prenatal diagnostic procedure prior to the 14(th) week of gestation. Over 2 decades of experience have demonstrated the accuracy, efficacy and safety of CVS. The karyotype is identical to that of the fetus in over 98% of cases. In the remaining 1 to 2% confined placental mosaicism (CPM) occurs. Current knowledge of the etiology of CPM allows accurate clinical interpretation, the recognition of cases of fetal uniparental disomy, and identification of pregnancies at risk for poor perinatal outcome. Prospective comparative studies have demonstrated that with equally experienced operators, CVS and second trimester amniocentesis have similar procedure induced miscarriage rates. When CVS procedures are performed after 10 weeks gestation, no increased risk of fetal anomalies has been demonstrated.