Strechmarks depend on the individual. If your skin has poor elasticity then you are bound to get them. Lotion can help with the itching and perhaps minimize the appearance by keeping the top layer of skin soft and flexible. I don't believe it can actually prevent them from forming though. They are supposedly hereditary, which is a ligit theory if elasticity is genetic. However, there are women out there (myself included) that were the first in their family to get strechmarks. I believe that has to do with how much weight you gained during your pregnancy. If your mother didn't have them and took care of herself, yet you ate terrible and gained 65 lbs, then perhaps you have a better chance of getting them considering you pulled your skin out further then hers was.
I don't think they're preventable. I believe they can be controlled with a healthy weight gain though and maybe (I stress maybe) minimized with a good lotion that can keep the top layer of your skin healthy. However, just to add my two cents in about using lotion, I applied cocoa and shea butter to my skin daily long before I ever became pregnant and throughout my entire 7.5 months and still got marks.