Screening for fetal anomalies by ultrasound at 14 and 21 weeks.

A prospective study of screening for fetal abnormalities and chromosomal defects was carried out by ultrasound examination at 13-15 weeks of gestation and 20-22 weeks; the first scan was performed transvaginally and the second transabdominally. During a 4-year period (1991-95), 3490 unselected consecutive pregnancies with a total of 3514 fetuses were examined. There were 21 chromosomally abnormal fetuses, including ten with trisomy 21, and, in 19 (90.5%), fetal defects were detected at the first and/or second ultrasound examination. The most effective marker for chromosomal defects was increased nuchal translucency thickness (> or = 4 mm) at the 13-15-week scan, which was present in seven of the ten fetuses with trisomy 21 and in six of the 11 with other chromosomal abnormalities.