I used Bio Oil throughout my pregnancy in 2008 to try and reduce the apperance of the stretch marks on my hips and I have to say they are less noticeable now, but i don't think it actually prevents getting stretchmarks.
I was one of the lucky ones to not get them over my stomach, but i did get them everywhere else in my pregnany in 2004.. hips, thighs, boobs, etc.. but that was mainly down to gaining 5 stone oops. They have all turned to silverish now though.