I am 19 weeks and 5 days and I went in for my 20 week ultrasound and they told me my baby cerebellum was smaller than normal and I have to go get an ultrasound by a specialist. What does this mean?

At this point, it just means your doctor wants someone who is more familiar with the brain to take a look. It doesn't necessarily mean there is a problem, but that he wants to make sure someone more qualified is looking at it. That being said, There are a variety of conditions that can cause a smaller cerebellum, including Downs. But there is no point in worrying over something that has not been accurately detected. This is the reason, most likely that the doctor didn't discuss it more with you. A stressed mama is a stressed baby, and that isn't good for anyone. :)