About 70 to 90 percent of expectant women wind up with stretch marks, squiggly scar tissue that occurs when the skin's elastic fibers stretch and break, says Mary Lupo, M.D., asso­ciate clinical professor of dermatol­ogy at Tulane University Medical School. Typically the marks show up as reddish-purple striations on the abdomen, hips, buttocks, upper thighs, and breasts during the third trimester, when skin has expanded most. In a year or so they fade to sil­very white, but never disappear.

    Until recently there wasn't much you could do about them; cocoa butter and other moisturizers don't penetrate the skin deeply enough. Now Retin-A, a prescription-only topical acne drug, is proving effec­live. (Retinol, an over-the-counter collsin, won't work as well.)

    A study at the University of Michigan Medical Center found that nightly applications of 0.1 per­cent Retin-A cream diminished active (reddish-purple) stretch marks after six months in 80 percent of the patients. Stretch marks increased or stayed the same for nearly all those treated with a placebo. "I've seen similar results with my patients," says Dr. Lupo. "Most get at least fifty percent improvement; in some the marks disappear. It works for light or dark-skinned women."

    Researchers speculate that tretinoin, the active ingredient in Retin-A, stimulates production of elastin and collagen, which help keep the skin supple and firm. "This new, healthy tissue buries the mark," says Dr. Lupo. "Stretch marks won't reappear when you stop using Retin-A, unless you get pregnant again or gain a lot of weight.

    But it takes about six months of daily use for significant results to show. For best results, the marks should still be reddish-purple when treatment begins. "Once they're established and turn white, Retin-A will have little effect," says Whitney Tope, M.D., assistant professor of dermatology at tl}eUniversity of Minnesota School of Medicine.

    What works on older stretch marks? New research suggests that a combination of 20 percent glycolic acid and either Renova (an emol­lient, weaker form of Retin-A) or a 10 percentvitamin-C serum (which helps in collagen formation) can diminish stretch marks, even those that are decades old. In a study by David McDaniel, M.D., assistant clinical professor of dermatology at Eastern Virginia Medical School, patients saw a 40 to 50 percent improvement after three months.

    As with Retin-A, see your derma­tologist if you want to try these treatments. (Over-the-counter gly­colic acid can be less effective and harsher on skin than buffered prescription strengths.) If you're pregnant or nursing, though, experts advise skipping Retin-A or Renova altogether. Although no studies have shown a risk, theoretically they can pass into your bloodstream and then to your baby.

    To minimize skin irritation, use these treatments before bed and wear loose pajamas, says Dr. Lupo. For a large area, like the lower abdomen, use enough to cover two or three fingertips and rub in well. Because skin can turn red and scaly during treatment, Dr. Lupo advises washing in tepid water with soapless cleanser and patting yourself dry.

    If you become pregnant again, you can try to limit stretch marks. Experts suggest gaining only the recommend­ed amount of weight, exercising reg­ularly to promote skin elasticity, and massaging the affected area (it increas­es blood flow to the skin, which can boost collagen production). Applying topical vitamin C to stretch-mark­prone areas may also help if it's strong enough, says Dr. Tope. Ask your dermatologist to suggest a product. -DONNA CHRISTIANO

The Laser Alternative

    Laser treatment, faster and less irritating than Retin-A, can even diminish old stretch marks. "The laser vaporizes residual blood vessels that cause redness and stimulates collagen production to plump up the skin," explains Trna Alster, M.D., director of the Washington [DC] Institute of Dermatologic Laser Surgery.

    The treatment itself is relatively pain-free and takes a few minutes to a half hour, depending on the size of the area. In most cases one session will fade 50 to 60 percent of a stretch mark, but you may need up to three sessions to treat all areas, such as abdomen, buttocks, and thighs. You can ultimately see up to a 90 percent improvement in the marks, says Dr. Alster. Afterward, your skin may be slightly bruised for three to ten days, and it takes about a month to see complete results.

    A $150 laser skin test can determine if you're a good candidate for the surgery. Sessions range from $500 to $600. (This is a cosmetic procedure, so most insurance won't cover it.) While you can have treatment soon after giving birth, wait a few months to see if stretch marks fade on their own. -KATE DEELY