Fetal cystic hygroma is a congenital malformation of the lymphatic system. The lymphatic system is a network of vessels that maintains fluids in the blood, as well as transports fats and immune system cells. Cystic hygromas are single or multiple cysts found mostly in the neck region. In the fetus, a cystic hygroma can progress to hydrops (an excess amount of fluid in the body) and eventually lead to fetal death. Some cases resolve leading to webbed neck, edema (swelling), and a lymphangioma (a benign yellowish-tan tumor on the skin composed of swollen lymph vessels). In other instances, the hygroma can progress in size to become larger than the fetus. Cystic hygromas can be classified as septated (multiloculated) or nonseptated (simple). Cystic hygromas can occur as an isolated finding or in association with other birth defects as part of a syndrome (chromosomal abnormalities or syndromes caused by gene mutations). They may result from environmental factors (maternal virus infection or alcohol abuse during pregnancy), genetic factors, or unknown factors. The majority of prenatally diagnosed cystic hygromas are associated with Turner syndrome or other chromosomal abnormalities like trisomy 21. Isolated cystic hygroma can be inherited as an autosomal recessive disorder. Fetal cystic hygroma have being treated with OK-432, a lyophilized mixture of Group A Streptococcus pyogenes and benzyl penicillin, and with serial thoracocentesis plus paracentesis.