I have to say I would wait until your 18 week scan as well.
I was told that DS was a girl at 14 weeks and obviously it was wrong. It wasn't until the later scans that it could be clearly seen that he was a boy. I understand how your feeling, I was a little disappointed when we thought we were having two girls because we aren't planning anymore either.
But they were wrong and we got our pigeon pair. Try not to get too depressed yet because it's still early days.