I occasionally dip into a pregnancy message board these days (though I really feel comfortable only on IF boards), and I have noticed that people are always asking how they can prevent stretch marks - like whether certain creams work, whether diet makes a difference, etc. I consulted What to Expect When You're Expecting on this topic, and it says that it's either genetics or the result of being well hydrated and having a good diet.
In my opinion, this is a load of hooey - I think it's all genetics. And I will tell you why - I am the best hydrated person I know, and I already have a shitload of stretch marks. I have them on my hips. I have them on my arms. I have them on my inner thighs and my buttocks and my boobs and my biceps. I may have them in places that I can't see. My body looks like a bobcat swiped it in multiple locations decades ago. In reality, they were all the result of a rather drastic growth spurt (seven inches of height in five months) when I was in the 12/13 age range. I remember noticing them in my mid-teens and asking my doctor, and he advised me of the proper terminology.
So if I am lucky enough to continue with this pregnancy until the end, then I know I will have a few more to add to the collection. (After all, I need some for my stomach.) For those of you who might be incredibly worried about this (oh, wait - only a fertile would worry about something like this), I am here to tell you that they do fade away to silvery streaks that are barely noticeable. And you could always claim you got them while wrestling a hungry bobcat away from a freshly born litter of puppies.