First-trimester ultrasound.

There has been an increase in the use of fetal ultrasound in the first trimester. This article reviews the published literature with emphasis on fetal nuchal translucency (NT). When measured according to well-defined guidelines, increased NT identifies about 80% of chromosomal anomalies for a false-positive rate of about 5%. Increased NT may indicate the presence of structural defects or genetic disorders, and it is relatively frequently seen in monochorionic pregnancies prone to develop twin-twin transfusion syndrome.