Is there anything that can be done during or prior to the pregnancy to reduce patches of dark skin?

No. The hormonal changes of pregnancy can cause lots of skin changes and there is not much you can do about them. Melasma refers to dark blotchy skin changes on the face sometimes called the mask of pregnancy. It is perfectly natural. You should wear sunscreen when outside and use moisturizer to keep the skin clean and healthy.
Sunscreen. I am guessing you are talking about melasma as hormonally induce darkening of the skin. There are several natural products that you can use such as soy as in aveeno (oatmeal) radiance, vitamin c serum such as in skinceuticals; i wouldn't do any bleaching creams during pregnancy. The best treatment is sunscreen with titanium and zinc not a chemical sunscreen. The higher the spf the better for melasma.