Personally i don't think they are harmful to a baby cause if they are required then that's the priority. My baby has a hole in her heart and i also have a history of placenta abruption therefore i have scans fortnightly. I wasn't exactly comfortable with the huge amount of scans i'm having (and they are extensive, long scans) but i'm in very good hands, seeing well-reputed specialists so i trust that they are doing what's best for my baby.