I rather like the expression 'baby shower'. I imagine a flock of obliging storks arriving with a mass delivery.
And 'washroom' for toilet is good too.
'In back' is strange. What happened to 'the' in there?
I do find 'ass' for arse ambiguous. It could cause all sorts of misunderstandings with donkeys and bottoms. (Jesus springs to mind)
I've never been to The States. I think I'd like it, but I'd need a phrase book.