One of my friends said that with their first, they decided to wait and see, and I thought it was a sweet reason why. My friend said that she felt connected with the baby, felt it grow and move inside her through the pregnancy, and she wanted her DH to have the special moment of seeing the bub when it entered the world, and letting her know if it was a little boy or girl.
I think we'd also wait, I'm a bit worried about the scans getting it wrong (it happened to someone else I know recently: first scan boy, second scan girl (born a girl)...