Cystic hygroma refers to the thickening of skin along the entire length of the fetal back as well as the visible division, or septations, between the skin and tissue. Septated cystic hygroma occurs early in pregnancy and is diagnosed in the first trimester. This condition is a separate diagnosis with a different prognosis than cystic hygroma diagnosed in the third trimester.
Cystic hygroma is different than increased nuchal translucency, as increased nuchal translucency occurs at the base of the neck only. When compared to nuchal translucency, cystic hygroma is five times more likely to be associated with aneuploidy, twelve times more likely to have an associated diagnosis of cardiac malformations, and six times more likely to result in fetal death.
Septated cystic hygroma has an incidence rate of 1 in 300 first trimester pregnancies, with 50% of cases being associated with aneuploidy such as trisomy 13, trisomy 18, trisomy 21 (Down syndrome), and Turner syndrome. Half of all cases of cystic hygroma with no chromosomal abnormalities have additional structural anomalies such as hydrops fetalis, skeletal dysplasia, and cardiac malformations like hypoplastic left heart syndrome, tetralogy of fallot, and ventricular septal defect.
Cystic hygroma, when diagnosed in the first trimester, has a high mortality rate, with 1 in 4 cases of isolated cystic hygroma resulting in intrauterine death. Only 15 to 20% of all cases of cystic hygroma result in the birth of a healthy baby.
The recurrence risk depends on the underlying cause of the diagnosis; therefore, prenatal genetic testing is recommended.