I found out that I was having a boy at 15 weeks:) I had a 3d/4d ultrasound and he was spreading eagle for us:)) 2d would have made it hard tp tell I think
I never had an ultrasound at 16 weeks, only ever had 20 weeks u/s but i have heard that sometimes you can tell, others say you can't, just depends how good the u/s tech is i suppose!