It's hyperpigmentation from the increase in hormones from pregnancy. I had the same thing and they went away after giving birth. Avoid the sun, it makes it much worse, or use a natural sunscreen. There are all natural skin lighteners as well, that will help remove the excess pigment in those areas. Look into nutgrass and bearberry extracts. They are all natural, are sold in creams for reducing hyperpigmentation, and are ok to use on your face while pregnant. I wouldn't use anything with chemicals.