During these years, hormonal changes due to pregnancy or birth-control pills often produce brown patches known as melasma, says N.Y.C. dermatologist Anne Chapas. "It's difficult to treat, as the patches often reappear with the slightest UV exposure." Look for a treatment with resorcinol or hydroquinone. If you're pregnant, choose a formula with a natural brightener, such as licorice root.
Try: Clarins Vital Light serum, $85; clarins.com