LosingMyMindInGA is right. if it's a choroid type cyst it can and usually will go away.
If your worried about genetic disorders, it's only a real worry when your baby shows 2 or more markers for one.
Common genetic disorders are Trisome 21 (Down Syndrome), Trisome 18 (Edwards Syndrome), and Trisome 9.
My son TJ had T21 and had growth problems and heart defects, all markers for T21.
Both of my kids had choroid pleIxus cysts...I waited 8-10 weeks to have a re-scan and they were gone on both of them. They are both doing very well. DS is almost 4 and DD is 9months :-)
It depends on the type of cyst. If it's a choroid plexus cyst and there are no other markers/risk factors then chances are extremely high that it will resolve itself within a month or two. with my 2nd daughter I was told she had a right choroid plexus cyst just under 1 mm in her brain. It was found during my 20 week anatomy scan. The docs assured me that becuase there were no other markers for genetic disorders that it would go away. 8 weeks later at my next ultrasound it was completely gone. She is now a healthy, rambunctious 7 yo.
Honestly cysts usually go away. When I was 20 weeks they told me my baby was healthy but they found a cyst on her brain. She told me it could be nothing but it also could mean that my baby had Trisomy 18 which is really serious and is the baby usually doesn't live after birth. She said she HAD to tell me the worst but she said that it is actually very common. I went baack in 4 weeks and the cyst had disappeared. My daughter is 13 months and perfrectly healthy. Did the doctor see any other markers?