Hi, I haven't been on here to update for a while!
My son was diagnosed with VM at 33 weeks. His right vent measured 11.5mm, he had a scan right after birth and it was 12.6mm. The in uteruo scans are not 100% accurate as you can imagine how small 1mm Is so the margin for error is massive which is why you get such varying numbers each time it's measured.
Cut a long story short, my son is almost 5 months old now and he's just amazing!
He smiles, laughs, rolls, he sits up with support, plays with toys (absolutely loves grabbing and holding toys) he eats solids already and is doing really really well.
If you look back through the posts on this thread my story is amongst it.
Please please do not stress over measurements of 10 and 11mm, the stats are that your baby has a 97% chance of being totally fine! And from 12-15mm it's 77%.
Poster with the third ventricle enlarged, I don't have an answer for you but, you are one of quite a few people I've heard with the 3rd vent problem. If you go into the Hydrocephalus group on babycentre, the ladies there know lots about VM, hydro, and other brain anomalies. As I said please don't worry about 11 and 12,13 mm. I know ladies with babies who's vents were in the 20's at 20 weeks. And by birth were 70mm, YES 7cm and their babies are developing totally normal!