My OB told me the darkened areas appearing on my face, the "mask of pregnancy," would go away after my babies were born, especially since I have fair skin (the mask affects darker skin people more often than fair skinned). Guess I’m the exception... while the pigmentation has faded, I can still see dark areas around my eyes, and it’s been five years. I’m thinking this is as good as it’s going to get!
Its scientific names are chloasma or melasma. Portions of sun-exposed skin, mostly the face, can darken during pregnancy due to an increase in hormones that effect the production of melanin (pigmentation) in your skin. This happens on any sun exposed areas, arms, neck, etc., but the face is the most common. This darkening of the pigmentation can also affect the color of your skin in other areas, such as your nipples and genitalia.
You may also see a darkened line appear from your belly button to your pelvic bone. This is called the linea negra. Again, completely natural, and will normally fade after you’ve given birth. (While I had the mask, I did not have the linea negra... go figure).
While you can’t prevent these pigment changes, you can decrease the effect by wearing sunscreen. The melanin is produced to protect your skin from the sun, so the mask will darken even more if you don’t slather on some SPF 30.
While my “mask” is still slightly visible, it doesn’t bother me. I have a good friend, (darker skin), who had a much more noticeable amount of pigmentation, and several months after her baby was born, she saw her dermatologist who prescribed a skin bleaching cream. It worked well for her, but I’ve sort of grown attached to my mask, so I’m going to leave it be.