I wouldn’t say it’s rude to send an ultrasound photo as a Christmas card. I’m not sure anyone here is saying that.
I believe some people, like me, are saying this is something I wouldn’t do. I consider it too personal, unnecessary, a little off-putting – that’s not the same thing as being rude, at least not to me. I think “rude” is too strong a word for this.
In deciding what to put into or on a Christmas card, I’d be asking myself “Is this something others would enjoy?” The sonogram wouldn’t pass that test for me.