Stretch marks are small scars that take place in the dermis, the tissue that lies just beneath the most superficial layers of skin. When scars form in the dermis, skin changes color and eventually softens.
Drinking collagen helps with strengthening connective tissues of the skin, and because it helps your skin turn over, it gets rid of that extra, uppermost layer!
When your skin becomes firmer, and smoother, it reduces the appearance of stretchmarks!
I also recommend scrubs -- my favorite one is to mix apple cider vinegar, coffee beans, coconut oil, turmeric altogether (it might not look or smell so great) but it has SUCH softening and skin-turning properties!