I had the same experience with my first pregnancy in 2003. At the 20 week ultrasound my baby also had bilateral choroid plexus cysts on the brain. I had a followup level 2 ultrasound at 24 weeks with the specialist and they had disappeared. My son is now 5 years old, and a very healthy, happy and intelligent boy at prepimary school. Try not to worry yourself about it. I know thats easier said than done when your pregnant. I hope this helps put your mind at ease.
Tania