Why do women undergo skin pigmentation changes during pregnancy?

Pigmentation changes are directly related to melanocyte-stimulating hormone (MSH) elevations during pregnancy. Some evidence suggests that elevated estrogen and progesterone levels cause hyperpigmentation in women. This is typically evident in the nipples, umbilicus, axillae, perineum, and linea alba, which darkens enough to be considered a linea nigra. More than 90% of patients have skin darkening. Facial darkening, called melasma, is a diffuse macular facial hyperpigmentation. When melasma occurs as a result of pregnancy, it is known as chloasma. This is due to the pigment being deposited in the epidermis itself. The distribution is usually malar but can be central or mandibular. Ultraviolet light exposure intensifies melasma and appropriate sunscreen decreases the effect. Because it is related to the hormones of pregnancy, it lessens with delivery.

Other pigmentation changes, such as palmar erythema, pseudoacanthosis nigricans, vulvar or dermal melanocytosis, or postinflammatory hyperpigmentation secondary to specific dermatologic conditions of pregnancy, are fairly common as well.

Source: http://emedicine.medscape.com/