Am I in the wrong??

So I have an 8yr old daughter and I had an ultrasound done about a month ago and the lady said that she thinks it's a girl. She didn't give me a positive on is it or not she said I think. Am I wrong for wanting to have a first peek ultrasound to get the 100% assurance that it is a girl I'm kind of feel bad because my fiancé is excited about having his first girl. My 8year old is not biologically his so he is super excited