It would be unlikely that if pregnant her teats would become enlarged already. But that doesn't mean she is not pregnant. All dogs are different in their heat cycle. First they bleed and the bleeding slowly becomes a cream color and than they become fertile and so on. If you were planning on spaying her, its probably a good idea to do it now.
Oh and Villette, I also had that problem with my tits becoming enlarged when I was pregnant!! LOL