Women who are 35 years or older are offered invasive prenatal testing because of the increased risk of chromosomal abnormalities, especially Down syndrome. In an attempt to increase the number of Down syndrome fetuses being detected and decrease the number of invasive procedures being performed on pregnancies not affected with a chromosome abnormality, both biochemical and ultrasound screening methods are being studied and are summarized in this article. The ultrasound markers reviewed include increased nuchal thickness, increased nuchal lucency, shortened femur, shortened humerus, pyelectasis, hypoplastic ears, echogenic intracardiac focus, hypoplasia of the fifth middle phalanx, and echogenic bowel.