Why does the face skin hyperpigment during pregnancy?

Chloasma. Chloasma is a required hypermelanosis of sun-exposed areas occurred during pregnancy and it can affect 50-70% of pregnant women. It presents as symmetric hyperpigmented macules, which can confluent or punctuate. The most common locations are the cheeks, the upper lip, the chin and the forehead. The exact mechanism by which pregnancy affects the process of melanogenesis is unknown.