Google Choroid Plexus Cyst. I'm guessing that's what it is. My son has one on his brain and is otherwise totally normal. From what I understand, these cysts could be part of normal fetal development and disappear before birth. Or they could be totally normal babies with the cysts too. There was ONE study done that showed a WEAK link to the cysts and downe syndrome so Doctors tell us about them so we are informed. I found that reading people's accounts of having perfectly healthy babies with more than one Cyst in their body at ultra sounds helped to dispell any fear. If your doctor isn't concerned, try and relax too :) I know it can be hard to, I hope this helped!
When my sister was pregnant she was told her daughter had a cyst on her heart and brain.. And this was just 3 weeks before she was born. But my niece was completely healthy and is now 3 and still so perfect :)