[Prenatal diagnosis of trisomy 21].

Trisomy 21 is the most frequent chromosome anomaly found in the living newborns. Prenatal diagnosis by amniocentesis was limited until recently to older pregnant women. Maternal blood biochemical markers (alpha-fetoprotein, unconjugated estriol, human chorionic gonadotropin) and ultrasonographic signs (nuchal fold thickening, femur length to biparietal diameter ratio) can discriminate a group of higher-risk women even though they are not aged. Numerous factors need to be considered when establishing such a screening program including pre-analytical variables (gestational age, diabetes, smoking, race), analytical variables (choice of reagents, quality control) or post-analytical (result reporting, follow-up of abnormal results). Until now the study of these markers has been restricted to the second trimester but they could become useful earlier during pregnancy.