If you've got brownish patches on your lips, chin, or cheeks, it is probably melasma (also called chloasma). Women with a family history or darker complexions are more likely to experience melasma than fair-skinned moms-to-be. This splotchiness is likely due to fluctuating hormone levels during pregnancy, and usually fades after your baby's born.
There's not much you can do to prevent melasma, but sun exposure can make it worse. You should wear a moisturizer with SPF every day, and slather on sunscreen with at least SPF 30 if you plan to be outside for long periods of time. If your melasma doesn't improve after your pregnancy, talk to your dermatologist. He or she can prescribe a number of creams or medications that have been found to improve skin discoloration.