Ok, name changed for this cos I don't want a load of abuse for suggesting that this is 100% or anything...but...
I know a bit about science/dermatology and decided with my first pregnancy that I'd put some if this knowledge to use. It worked -I have absolutely no abdominal pregnancy stretchmarks.
Basically stretchmarks are related to what kind of collagen you have. Your type is gentically determined. Some collagen forms a pretty regular cross hatch pattern, nice and tight and dense. If you have this type you are much less likely to get stretchmarks and your skin probably doesnt get particularly cellulite-y when you get fat. With minimal care your skin looks good, basically. The other main type is a bit haphazard, less regular and more prone to stretchmarks and full on cellulite. The majority of caucasians have this latter type.
I have the latter type and have been very sensitive to the effects of female hormones. From puberty I developed stretchmarks all over my boobs, hips, thighs and even my bloody calves. As a result I can't go tightless in the summer and hate being seen in a swimming cossie, even though I'm a pretty average size 12. Knowing my mum and sisters are all the same and added a big stretch marky tummy pouch to the package with their pregancies I decided to make an attempt to see if I could possibly avoid this. After all, I'd need to rub some sort of moisturising lotion in, why not something that gave me the best chance of avoiding.
So, knowing a bit about collagen and the vitamins need to support it I decided on a regime of pure Vitamin C serum, pure Vitamin E serum plus collagen gel. I found three separate products (all available from QVC and a few other online shops - I used Alpha H vitamin serums and a SBC Collagen gel). Every morning and night I rubbed a couple of drops of each serum into my tummy, brushed my teeth and then rubbed in the collagen gel. That was it -I got no stretchmarks on my tummy.
The punch line is that I didnt rub it in anywhere else and got new stretchmarks on my boobs, bum, hips and thighs. Stupid me.
I don't claim to know anything about the safety of using vitamin serums in pregnancy. I could find no warnings against any except Vitamin A which wasn't one I thought would be useful anyway. Vitamin E has been used in skincreams for a long time and we tend to excrete any excess Vitamin C in pee. I wouldnt think it would get anywhere beyond the skin anyway. But the bottom line is I did my own research and if you wanted to do this you should too either by checking with the manufacturer or your health care professional :)