My 4th baby had this problem it was picked up at the 19wk ultrasound, I had to have a amino(a needle through the side of my tummy) and the doctor doing it said my son had 12mm of fulid and the amount shouldn't be anymore than 10mm in the ventricles. They even said to start thinking about stopping the pregancy which freaked me out. My doctor was having confrence with lots of different doctors and discussed my case. I couldnt be given any answers but it wasn't much more than it should be so it should be ok. The doctor was concerened about the large head aswell, but all my kids have large heads lol. My son was born 2 weeks early he was 8pounds 11ounces he looked perfect but then had the newborn hearing tests and had problems with his hearing, it was due to fulid build up in his ears. He had grommets put in at 3.5months old to drain the fulid. He walked a bit later than my other 3 kids but other than that he is fine.  So i hope this helps and if anyone else can give more information i would love to read it as well. as i would like to know more myself.