[Prenatal diagnosis in the first trimester, whom and how?]

Since the last decade more screening tests for chromosomal abnormalities are available to all pregnant women in the first trimester. In the same way, more women above 35 years of age are now pregnant. Offering direct invasive procedures to this "high-risk" group should increase the number of fetal losses associated with amniocentesis or chorionic villus sampling (CVS). Different screening procedures could modify the background risk and thus the clinical management. The nuchal translucency (NT) measurement has been the most important factor to detect fetuses at risk of chromosomal abnormalities. A NT above the 95 centile can detect 70% of fetuses with Down syndrome with 5% of false positive rate (FPR). In addition to the NT, maternal serum markers as the free beta human chorionic gonadotropin (beta hCG) and pregnancy-associated plasma protein (PAPP-A) protein can improve the detection rate to 80% with a fixed 5% of FPR. Preliminary reports showed that the combination of NT, maternal serum markers and the evaluation of the nasal bone could detect more than 90% of fetuses with Down syndrome with 5% of FPR. The CVS performed after 10 weeks of gestation has the same risk as the amniocentesis in the second trimester. Placental mosaicism identified by the CVS might be associated with early growth restriction and placental insufficiency. The complete assessment must be offered to all patients at around 11-14 weeks. In cases when a CVS is performed the complete results can be ready in 72 hours.