I don't have much in terms of answers, but there's a post on another board here with people who have had pregnancy and babies like this. The answers and responses might help you a little, and you could post your concerns on that post as well. Good luck with your pregnancy!!
http://www.medhelp.org/posts/Maternal--Child/cystic-hygroma/show/159477?page=1
You could also message Megan0701. She seems to have a great deal of information on this and could (I hope) give you some good answers.
This is like a fluid collection like a sack behind the neck or something??
From what I understand there is a better chance for the baby if it develops after 30 weeks cos the chances of it being due to Chromosomal (spelling on that) are alot slimmer but it has to be said it can result in fetal death, not 100% of the time obviously but so you know it can happen, also a chance of it blocking the babies airwaves. There is also a chance that hydrops can develop aswell
How far along are you?
I'm not 100% on the info either but Thats my understanding of it but someone will correct me if I'm wrong anywhere I'm sure. Sorry you have to go through this x