Most fetuses with major chromosomal abnormalities have either external or internal defects that can be recognized by detailed ultrasonographic examination. These are defined as ultrasound markers for fetal chromosomal defects. In case of trisomy 13, 18, Turner's syndrome and triploidy, what we would consider as ultrasound markers are often major defects. In contrast, in Down's syndrome fetuses the structural defects are subtle and often isolated. Ultrasound screening programs for this trisomy are based on a systematic search for specific markers. Chromosomal anomalies are more common in fetuses with multisystem malformation. In fact the risk of a fetal aneuploidy increases with the number of detected defects. Other important prognostic factors are maternal age and gestational age. For every single structural abnormality it is possible to estimate the specific risk of this being a phenotypic expression of a chromosomal defect according to whether it is isolated or associated to more structural defects. This risk will also be influenced by the maternal age and the gestational age.