It's funny, this is something that I'd never even thought about. I think last u/s we had they said baby. But with DS we said we didn't want to know, and at one point while they were checking things, they said to turn away from the screen, just in case baby moved and showed us some bits...

But I'm one to call inanimate objects "him" generally, and MIL told me after we had DS that I'd said it at one point. She thought we must have known. I didn't even realise!!