I've found that stretch marks get worse in the sun. The marks are actually little scars, and the skin is very sensitive to sun damage.
Back in August I had a breast reduction, and the scars were substantial. My surgeon recommended Mederma for the scars, and I can't tell you how effective that was! I'm incredibly impressed with the results. Today, a good portion of the scars are very faint to almost invisible. (In the first month or so, I looked like Frankenstein!) I used the Mederma religiously. I'm 45, and my skin is not as lovely and elastic as it was when I was younger, so I'm swearing by the Mederma. My surgeon says that he's going to make me his "poster patient" because my results were so good!
It's very pricey stuff, but a little goes a very long way. It also has a distinctive aroma, but that does fade after a bit. You might want to give it a try. I ordered my supply from Drugstore.com, since there always seems to be some sort of online coupons or discounts there. I know a few people who've used it on stretch marks and have had very good results, even when first using it several years after having their babies.