Melasma, also known as chloasma, are brown patches of hyper-pigmentation that typically occur on the face. Although melasma can affect anyone, it is most commonly seen in women. It is believed that estrogen, stimulated by pregnancy, birth control pills and hormone replacement therapy, causes skin to produce greater amounts of melanin in the skin. Sun exposure can make melasma worse, so the first line of defense is a broad-spectrum sunscreen like Neutrogena's Ultra Sheer line of sun protection products. I love this formulation because it comes in both a cream and a spray. 

If you already notice developing melasma there are several treatments that can be used in order to fade darkening spots. Topical therapies include the use of hydroquinone, which inhibits an enzyme involved in the production of melanin, tretinoic acid, which increases skin cell turnover, and azelaic acid, which decreases the activity of melanocytes. Other treatments include: microdermabrasion, chemical peels, or laser treatment which all facilitate the fading of melasma. You should consult your dermatologist to explore all of your options.

One thing to note: melasma is stubborn, so you may need to battle it with several treatments. So, buy a good sunscreen and a hat or else you may find yourself fighting a losing battle!