Skin lightening products contain chemicals called hydroquinone or glutathione that are used to bleach skin. They affect the enzymes in your body that produce melanin, and therefore block a natural process needed for your baby to have healthy dermis. Additionally, these ingredients may cause an allergic reaction in some people and should be avoided when pregnant or breastfeeding as well. The downside is that oftentimes, pregnancy causes pigmentation issues. If you happen to experience this, consult your physician and don't take any action until after your pregnancy.