Cystic hygroma is a condition where a mass appears on the head and neck areas. This defect usually occurs from birth. This condition occurs when the baby is still inside the womb. Cystic hygroma develops from the bits of material that carry white blood cells and fluid. This material is known as embryonic lymphatic tissue. Once the baby is born, the cystic hygroma looks like a soft bump under the skin. The bump may not be noticed at birth. They grow as the child grows and in some cases go unnoticed until the child gets older.