thick heart muscle?

Dr. just reviewed my 20 week sono and said that baby has thick heart muscles. He said this is only an indication of possible down's syndrome if there are other signs on the sono (he said there were none). He kept reassuring me that the baby was normal and that he was just obligated to mention the thick heart muscles. He said I could take an additional test (can't remember the name of it though) if I wanted to but he said that he wouldn't recommend it because it wouldn't tell us much anyway.

Just wish he didn't plant this seed in my head. I thought I was done with the Down's worry after my AFP came back fine.

Anyone familiar with this?