Melasma on the face is usually set off by a trigger. The AAD says sunlight, taking birth control pills, and even stress can trigger melasma.
Triggers the AAD names
The AAD also names pregnancy and medication as examples of a trigger that causes melasma.
What you can check
Look at which of these apply to you: sun exposure, a pill you take, a pregnancy, a stressful stretch. Writing them down gives a dermatologist a better starting point.
Patient pages on melasma
Two patient pages explain the condition: the MedlinePlus melasma encyclopedia entry and the AAD melasma overview.
New medication, pregnancy or a sunnier season
The cause question gets clearer when a trigger changes. A new medication, a pregnancy, or a sunnier season is the time to compare how the patches look before and after.