Sonographic markers of fetal aneuploidy

Ultrasound can detect abnormalities in some fetuses with chromosome abnormalities.(74, 76) Positive ultrasound findings increase the risk of fetal aneuploidy whereas a normal ultrasound reduces the risk. To what degree the risk is increased or decreased depends on a variety of factors including the type of chromosome abnormality, the gestational age at the time of ultrasound, the criteria used for a positive scan, and the quality of ultrasound. In discussion of ultrasound findings, we distinguish major or structural defects from nonstructural findings or ultrasound markers (Table 1). A major abnormality (e.g., cardiac defect or omphalocele) can be confirmed after birth or at autopsy, and carries a greater risk for fetal mortality and morbidity. Detection of a major abnormality should initiate referral to an obstetric center for detection of additional anomalies and for chromosome analysis. More often, we are faced with one or more ultrasound markers, especially in fetuses with trisomy 21. Unlike structural anomalies, ultrasound markers are insignificant regarding outcome themselves, are nonspecific and may be seen in normal fetuses, and are often transient. Example of minor abnormalities include nuchal translucency (NT) in the first trimester, and nuchal thickening, echogenic bowel, shortened extremities, renal pyelectasis, or echogenic intracardiac foci (EIF) in the second trimester. It is helpful to be aware of typical phenotypic expressions of the common trisomic conditions (Table 2). As with other chromosome abnormalities, however, the phenotypic expression can vary widely in an individual fetus. Also, the type and frequency of ultrasound findings vary with gestational age. The following sections discuss ultrasound findings of the first and second trimesters separately. We have emphasized ultrasound markers because these are problematic in everyday practice.