Hi phillipa, so sorry you are going through this. Happened to us a few years back. My baby's cystic hygroma was much more pronounced, 8mm at 12 weeks, and extended all the way down the back. A consultant did a detailed cardiac scan which may be the way for you to go, since cardiac abnormalities may cause the fluid build up which causes the cystic hygroma.
In our case, although no cardiac defects were found at that scan, there was clearly something going v wrong with our baby's development early on in gestation to have such a huge cystic hygroma. We were given v poor statistics re the baby surviving in utero and being delivered safely. Therefore our decision to end the pregnancy was the absolutely right one for us, although it was still extremely sad.
We had some tissue tested for chromosomal abnormalities, and we were reassured that nothing was wrong here. It gave us the courage to try again. The trauma of having that baby would have prevented the possibility of having another healthy one for us.
I'm not sure if the prognosis is necessarily so dire in smaller ones. This is something, as well as cardiac scanning, and chromosomal testing, that you need to ask your consultant about.
I think that what you have to do is gather your thoughts and think about how a worst-case scenario is going to impact you and your family unit. And whether you want to brave that.
I am sorry I cannot give you any hope and I wish you and your partner lots of strength. Go easy on yourself.