Stretch Marks!

I'm pretty active and toned and I was in shape before I got pregnant. I thought I was in the clear after hitting my 30th week mark but apparently that's not the case. Although very subtle, I've notice stretch marks above my navel where my belly ring used to be and on the right side of my belly where my little guy likes to lay his head. As of right now, I'm 33 weeks and 6 days today. I've been eating healthily(sometimes) LOL, somewhat maintaining physical activity for 30 minutes a day, and lathering with thick body moisturizing creams since before I was even pregnant. So, why are they revealing their ugly heads? I know genetics play a huge role but my mom was not in the shape I was when she had us. So my question is, does being in shape have anything to do with the prevention of stretch marks or is it ALL up to genetics?