The most effective sonographic marker of trisomy 21 and other chromosomal defects is increased nuchal translucency (NT) thickness at 11-14 weeks. Extensive studies over the last decade have examined the methodology of measuring NT, the development of the necessary algorithms for calculating the individual patient risk for trisomy 21 by NT in combination with maternal age and with various maternal serum biochemical markers, and the performance of this test. Another promising marker for trisomy 21, both in the first and second trimesters, is absence of the fetal nasal bone. There is also an extensive literature on the association between chromosomal abnormalities and a wide range of second trimester ultrasound findings. However, there are very few reports that have prospectively examined the screening performance of second trimester markers. This article reviews the association between sonographically detectable fetal abnormalities and chromosomal defects, and examines the value of incorporating these defects in screening policies.