Hyperpigmentation is a dark discolouration on the skin due to the overproduction of melanin. Factors like ultraviolet (UVA) rays can overstimulate melanocytes, the pigment-making cells that produce melanin to protect the skin. Examples of hyperpigmentation are sun spots, scars after an acne breakout, or even brown patches. Hyperpigmentation can appear decades after excessive exposure to the sun during childhood or teenage years. 

Melasma, often referred to as the "mask of pregnancy," is a brown darkening of facial skin, usually occurring on the cheeks, forehead and upper lip. MElasma occurs from a combination of factors, such as exposure to the sun and an increase in the female hormones of estrogen and progesterone. It usually fades during the winter months in northern climates but returns in the spring. It also tends to fade after pregnancy or with the cessation of oral contraceptives or hormone therapy. 

Melasma often affects women who:
have dark skin
take oral contraceptives of hormone therapy
are pregnant