Screening for adverse pregnancy outcome at early gestational age.

In the past two decades second-trimester maternal serum screening for Down syndrome has been the most common strategy for prenatal diagnosis of chromosomal aneuploidies. More recently, screening for and diagnosis of chromosomal abnormalities have increasingly been performed in the first trimester. With improvements and technological advances in ultrasound, it is now possible to identify many fetal anomalies at 11-13 weeks of gestation.