Tretinoin, or Retin-A, loosens the keratin in the outer layer of skin to remove dead cells. Though derived from vitamin A, it is much more effective than the naturally occurring vitamin because it thins the outer layer of skin in order to penetrate it. It gets into the dermis, where stretch marks occur, increasing collagen production. (This makes it more effective than alpha hydroxy acids as well.) The result is improved appearance and texture of the skin, which can affect the appearance of stretch marks. Consult your physician about appropriate dosages. Be aware that stronger dosages, though more effective, can cause stinging, swelling and redness, and an extreme sensitivity to sunlight. Skin thickening is another possible side effect, due to the heightened collagen production. Tretinoin is most effective in stretch mark treatment when used early.
Tretinoin is classified as a class-C drug for pregnancy, which means the risks of using it during pregnancy outweigh the benefits. (No one knows the effect of the product during nursing either.) Because tretinoin works best during the onset of stretch marks, it's not a recommended course of therapy for pregnant women.