bottom line is YOU ARE THE BOSS!! you have decent concerns and need an u/s. that's a fact. regardless of whether anyone here agrees w/ you getting one. so, here's what you do. put down the conditions. Before he starts, say, I have some things we need to go over.
check for omphalocele, # of fetuses, position of placenta and that's it! (if that's it). Make this the shortest u/s of your career. mine was happy to do that 7 minutes.