A cystic hygroma is an abnormal growth that usually appears on a baby's neck or head. It consists of one or more cysts and tends to grow larger over time. The disorder usually develops while the fetus is still in the uterus, but can also appear after birth. Cystic hygromas are benign, but can be disfiguring. It is a condition which usually affects children; very rarely it can be present in adulthood.