Screening for the aneuploid fetus

Familiarity with the specific sonographic features that are characteristic of particular syndromes aids the clinician performing ultrasound examinations, particularly with respect to risk assessment and the decision-making processes regarding which pregnancies warrant invasive fetal testing. For example, the recognition of polydactyly should encourage a detailed survey of the systems usually affected in trisomy 13, such as facial abnormalities. Even when findings do not point to a specific syndrome, certain general sonographic features strongly suggest a substantive risk of a fetal chromosomal disorder and require the performance of genetic counseling and further definitive testing.