Unusual skin pigmentation is sometimes a sign of pregnancy, although this usually occurs on the face (chloasma). In early pregnancy, the nipples can also become larger and darker, too.

Uneven darkening of patches of skin can occur with a variety of skin conditions, including eczema, psoriasis, pityriasis versicolor (fungal skin infection) and as a result of long-term exposure to the sun. Darker and more permanent darkening of patches of skin may also be caused by increased pigment production, as in freckles and moles. It is difficult to know exactly what this is without examining you, so you will need to see your doctor to get the skin changes checked.