Detection of fetal structural abnormalities with US during early pregnancy.

Ultrasonography (US) is performed during early pregnancy for dating, determination of the number of fetuses, assessment of early complications, and increasingly for evaluation of the fetus, including measurement of the thickness of the nuchal translucency (NT). Measurement of NT thickness between 11 and 14 weeks gestation, combined with maternal age and maternal serum biochemistry, can be an effective method of screening for trisomy 21 and other chromosomal abnormalities. Furthermore, an increased NT thickness in the presence of a normal karyotype is associated with an increased frequency of structural defects and genetic syndromes. Therefore, this finding is an indication for a more detailed anatomic survey of the fetus. Besides nuchal abnormalities, a wide range of other congenital anomalies can be diagnosed with US at 11-14 weeks gestation, including defects of the central nervous system, heart, anterior abdominal wall, urinary tract, and skeleton. The anatomic survey can be performed with a standardized protocol by using transabdominal US and, when necessary, transvaginal US. A thorough knowledge of the US features of normal fetal development is necessary to avoid potential diagnostic pitfalls.