post baby bellies

This May sound vain but I'm curious to know why some women have stretch marks and flabby skin while others look like they just left the gym a few weeks after having a baby. Why is this?! I'm trying to conceive my first baby and fitness is very important to me.
I know stretch marks are largely hereditary but I wonder what "real" post baby bellies look like. 
Tell me about yours! Are you happy with the way it looks post baby?