Melasma is the term used to describe the formation of irregular hormonal pigmentation on the skin. Melasma appears as "mask like" patches on the cheeks, temples, upper lip and forehead and is colloquially known as "pregnancy mask". Hormonal changes in the body temporarily stimulate increased melanin production and make the skin more photosensitive. It is also known as cholasma.
Melasma is thought to be caused by increasing levels of the hormones oestrogen and progesterone, which stimulate melanocytes (the cells that produce melanin) resulting in increased production of the protective pigment. Melasma is aggravated by exposure to the sun.
Melasma is most common on the face and appears as "mask like" (normally symmetrical) patches on the forehead, temples and cheeks. You can also get it on your body but this is less common.
Because melasma is a hormonal condition it is most common in pregnant women or women on the pill. As such most women who get it are between the ages of 20 and 50. It is also more common in people with darker skin types.