I completely feel your anquish. The doctors told me with both my children that their heads were large and that could be a marker of Downs. Neither of them have Downs Syndrome. There are other more significant markers than a round head. Did they look at the fingers? A shorter pinky is more clear marker than the head and that isn't even definitive. I know there are other more clear markers too. Do you or the father have a round head? Maybe it is genetic. When the doctors told me about the head size of my babies, I simply said, "look at the size of my husband's head. I guess they'll just need big hats!"
They do have to tell you these things, but since everything else was normal I'm sure there is nothing to worry about! A round head is a VERY soft marker. If they really thought it could be Downs they would not have told you it's highly unlikely anything was wrong.
I wish you luck with the rest of your pregnancy and try to enjoy it as much as possible!