Originally posted by wolfsoul
You can't actually tell from the nipples in this stage; it's only accurate around 14 weeks. It's difficult to tell even at 3 weeks because the males shouldn't have descended testicles yet.

Here are a couple pics of rats, I couldn't find any of mice but it's pretty much the same thing:
http://www.ratsandmice.com/femalebaby_marked.JPG
http://www.ratsandmice.com/malebaby_marked.JPG

Also, you shouldn't seperate them now. They should be together another two weeks.
You mean 14 days.