Prenatal detection of fetal anomalies is one of the major goals of obstetrical ultrasound. The primary reason is the options that are often offered to the family and caregivers from therapy in selected cases to special care at delivery to termination of the pregnancy. An important aspect of the diagnosis is to determine whether the anomaly is expected to be lethal or associated with severe physical or mental impediments. This goal is often difficult to accomplish without a clear diagnosis. A systematic approach is essential when an abnormality is first identified sonographically to help the practitioner discover certain patterns of associated defects. The use of this logical and stepwise strategy facilitates arriving at the correct diagnosis of specific syndrome by taking all anatomic findings into account. This process focuses on first pinpointing a key or sentinel feature specific to each syndrome and which can anchor the diagnosis.