I'd ask for a genetic test to be ran before doing anything. Most cystic hygromas are accompanied with bad genetic problems. Like the kind that can cause slow painful deaths, miscarriages, or no quality of life what so ever. If there is no genetic issue then it's just a fluid filled sack on the baby's neck or head that can be drained. However they only get larger from here so there can be problems anyways. If it were me and the genetic tests came back letting me know my baby was hurting in me, would be doa, or have absolutely no quality of life, heart beat or not I'd let it go. It would hurt me too bad to put it through that type of suffering.