Down syndrome: prenatal risk assessment and diagnosis.

Down syndrome (trisomy 21) is the most commonly recognized genetic cause of mental retardation. The risk of trisomy 21 is directly related to maternal age. All forms of prenatal testing for Down syndrome must be voluntary. A nondirective approach should be used when presenting patients with options for prenatal screening and diagnostic testing. Patients who will be 35 years or older on their due date should be offered chorionic villus sampling or second-trimester amniocentesis. Women younger than 35 years should be offered maternal serum screening at 16 to 18 weeks of gestation. The maternal serum markers used to screen for trisomy 21 are alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin. The use of ultrasound to estimate gestational age improves the sensitivity and specificity of maternal serum screening.