Melasma (muh-LAZ-muh) is a common skin problem. It causes brown to gray-brown patches, usually on the face. Most people get it on their cheeks, bridge of their nose, forehead, chin, and above their upper lip. It also can appear on other parts of the body that get lots of sun, such as the forearms and neck. Birth control pills, pregnancy, and hormone therapy can all trigger melasma. Stress and thyroid disease are also thought to be causes of melasma. Additionally, heat and/or sun exposure can cause melasma because ultraviolet rays affect the cells that control pigment (melanocytes).