I have no idea... stretch marks hereditary?

Everyone is different but stretch marks are hereditary. Chances are if the women in your family have them, you will get them too, regardless of weight gain. However, the number and severity will be different depending on how you take care of yourself. I gained under 10 lbs and still got stretch marks. They're not nearly as bad as all my family members (some of them have them covering their entire belly), I only have eight. And they can start showing up at any time. Some girls get them in their first few months when they aren't even "showing" yet, and sometimes they don't show up until right near the end (giving you a false sense of security that you're not going to get them!). Keep your belly well lotioned and your weight in check and you'll probably get a few... but they're not as bad as you think. I look at them like my little badges of honour for a job well done!