Neonates with trisomy 21 (Down syndrome) often have thickening of the soft tissues in the posterior neck. This has a prenatal analog, because thickening of these soft tissues (the nuchal fold) at approximately 16–20 weeks gestation has been found to be associated with trisomy 21. Forty to 50% of second trimester fetuses with trisomy 21 have a nuchal fold of ≥6 mm. When a thickened nuchal fold is found in a 16–20-week fetus, the parents should be counseled about the risk of aneuploidy.

The nuchal fold should be routinely measured on sonograms performed between 16 and 20 weeks gestation. The measurement is obtained on a transverse view of the fetal head in a plane that is close to axial but angled slightly coronal to include the cerebellum and occipital bone. The nuchal fold is measured from the external surface of the occipital bone to the outer skin surface. A nuchal fold measurement of 6 mm or larger is abnormal (Figure 9.1.1) and should prompt a careful sonographic search for other anomalies, especially those associated with trisomy 21.

A cystic hygroma is a uni- or multilocular subcutaneous mass filled with lymphatic fluid. It most commonly occurs in the posterior neck, although it can be seen elsewhere in the body. A cystic hygroma results from a localized area of lymphatic dysplasia, with dilation of, or leakage from, lymphatic vessels. Sometimes, the subcutaneous lymphatic dysplasia is widespread, with fluid collections or subcutaneous edema encompassing the entire fetal body (generalized cystic hygroma or lymphangiectasia). Even when generalized, the largest fluid collections are typically in the posterior neck.

Fetuses with Turner syndrome (45, X karyotype), trisomy 21 (Down syndrome), trisomy 13, and trisomy 18 have elevated incidence of cystic hygromas and generalized lymphangiectasia, particularly during the latter part of the first trimester and the beginning of the second trimester. In view of this, the sonographic finding of cystic hygroma should prompt genetic counseling. Regardless of the fetal karyotype, fetuses with severe generalized cystic hygroma have high mortality rates. While the cystic hygroma that occurs at the back of the neck is highly associated with aneuploidy, isolated cystic hygroma or lymphangioma occurring elsewhere in the soft tissues of the body is less often related to aneuploidy.

A cystic hygroma appears sonographically as a cystic mass in the subcutaneous tissues, generally in the posterior neck, often with septations (Figures 9.2.1 and 9.2.2). Cystic hygromas can also occur elsewhere in the body (Figure 9.2.3). Large focal cystic hygromas are sometimes called lymphangiomas (Figure 9.2.4).

With generalized cystic hygroma or lymphangiectasia, the entire fetus is surrounded by a hypoechoic halo, representing subcutaneous edema (Figures 9.2.5 and 9.2.6). There may be well-defined cystic spaces interspersed within the diffuse edema. In some cases, ascites, pleural effusions, or pericardial effusions may also be present (termed “hydrops” if at least two abnormal fluid collections are present).