I see that you posted this in a few places - I would also suggest that you try the pregnancy forums as they may have seen this come up before.
This is a patient forum - so there are no experts here.
http://www.medhelp.org/posts/Neurology/what-is-a-giant-cisterna-magna/show/478150
Was a previous question answered by a doctor - so it may set your mind at ease.
The only other issue that came up in my research was that there may be a possibility of Dandy-Walker syndrome when this is found so testing may be done now to rule it in or out.