I think the jury is still out on safety of repeated use of ultrasounds. I'm pregnant now and the pregnancy discussion forum I participate in has posters with various views and various advice from doctors on this. My own doctor told me they only schedule 2 ultrasounds during the pregnancy unless there are complications. She said she was just being prudent because there are conflicting opinions.
Our ob/gyn told us this 18 years ago! It's still a good precaution.