I found out at my 18 week US because I'm just incredibly impatient - though I was telling everyone it was a boy a month before then anyway lol I'd been having dreams that he was a boy so I just assumed anyway but it was nice having it confirmed
Next time I don't think I'll find out though.. the surprise will be nice
Not very helpful I know but the envelope sounds like a good idea just put it somewhere you can't reach without DH so you can't peek hehe then if in a week you do decide that yes you want to know - its there.