Sometimes there is a problem - my niece was sure she was having two boys when she was carrying twins 4 years ago, I told her sometimes the sonograms are wrong, but she told me the doctor was sure and so was she. Libby came out first.... with her brother following.
My doctor told me with each pregnancy that I was carrying boys and I had two girls - so if I had a reveal party it would have been another surprise. Sometimes they are wrong and it is best to wait. (We were so thrilled the doctor was wrong - we could never agree to a boy's name.)