What causes melasma?

Research has shown that when someone has melasma, the cells that give skin its color (melanocytes) tend to be more active, but the exact cause isn’t well understood. It may be that melasma develops when something triggers these skin cells, causing them to go into overdrive. Melasma is more common in women than men and hormonal changes, such as pregnancy and use of birth control pills, can often trigger this condition.