Hi, My Name Is Karen, My Daughter Is 21 Weeks

hi, my name is Karen, my daughter is 21 weeks pregnant and she went to the doctor today for an ultrasound. On the ultrasound they found a fluid blister on the baby's brian. They said 9 times out of 10 they go away they will do another ultrasound at 28 weeks if it is still there she will go to a high risk pregnancy doctor. Should we be worried what does this mean?