Cystic hygroma refers to the finding of marked skin thickening extending along the entire length of the fetus at early ultrasound examination (Benacerraf and Frigoletto, 1987; Langer et al., 1990; Thomas, 1992; Gallagher et al., 1999). This finding is to be differentiated from simple increased nuchal translucency in which skin thickening is noted at the posterior aspect of the fetal neck only. Additionally, cystic hygroma in the first trimester has clearly visible septations running transversely between the fetal skin and underlying subcutaneous tissue. It appears that the diagnosis of this finding early in pregnancy represents a completely different entity than the diagnosis of cystic hygroma in later pregnancy (Chapter 32). The latter condition is typically an isolated structural abnormality of lymphatic drainage and appears to have little or no association with chromosomal abnormality or other malformation.

The importance of this sonographic finding is its contribution to general population screening for fetal abnormalities and fetal aneuploidy in the first trimester of pregnancy. Given the pivotal role that nuchal translucency sonography at 11 to 13 weeks’ gestation currently plays in general population screening for fetal abnormalities (Chapter 2), the finding of cystic hygroma will likely become more common. Once identified it is clear that such pregnancies are at significantly increased risk for a range of adverse outcomes. This includes up to a 50% chance of fetal aneuploidy (Malone et al., 2005a). Amongst euploid pregnancies there is almost a 50% chance of cardiac malformation or other major structural abnormality. Additionally, about one in four pregnancies with cystic hygroma in which a fetal abnormality has been excluded will result in spontaneous intrauterine fetal demise. Ultimately, only 15% to 20% of all cases of cystic hygroma identified in the first trimester will result in the birth of a healthy neonate at term (Malone et al., 2005a).

Until recently it was difficult to assess the population prevalence of first trimester cystic hygroma as most studies have included only selected patients identified from high-risk referral centers (Bernstein et al., 1991). However, a recent general population screening study evaluated more than 38,000 unselected pregnancies from throughout the United States with first trimester ultrasonography. The prevalence of cystic hygroma in this group was 1 in 285 first trimester pregnancies (Malone et al., 2005a). It is therefore likely that in a busy clinical practice performing routine first trimester ultrasonography, septated cystic hygroma will be encountered frequently.

The key sonographic criteria for diagnosing cystic hygroma in the first trimester of pregnancy include the presence of a markedly enlarged nuchal translucency measurement that extends along the entire length of the fetal back, and in which septations are clearly visible in transverse section through the fetal neck. Figure 31-1 demonstrates a sagittal view through a fetus at 12 weeks’ gestation in which skin thickening is seen to extend from the fetal head to the base of the fetal spine. Figure 31-2 illustrates the septations that are clearly visible in a transverse view through the fetal neck. Such septations tend to be more easily noted using the higher frequency imaging provided by a transvaginal transducer. Figure 31-3 demonstrates a 3D ultrasound image of a septated cystic hygroma.