Funny you say that, my belly button did not pop out during my first pregnancy and during this one has not either.
I'm in my 30th week. I was told not everyones belly button pops out while pregnant and less likely for overweight.
Mine didn't either with my first pregnancy, although I have a deep belly button too, so that's probably part of it.
P.S. As a kid I always thought when the belly button popped it was a sign that the baby was ready - just like a turkey timer! haha! But if you've ever cooked a turkey, sometimes the bird is done before the timer pops! :)
mine never popped out with either pregnancy