Why do women get melasma?

Unknown. We still do not know why some women who are pregnant get melasma.
Great question. A tough but good question. It is thought to be caused by the stimulation of melanocytes or pigment-producing cells estrogen and progesterone. This can cause those cells to produce more melanin pigment when the skin is exposed to sun. Pregnancy can trigger this as can the use of birth control pills. It is a frustrating condition to have and to try and treat as there are no guaranteed treatments.