Will topical Tretinoin Cream (0.05%) make Previfem birth control less effective?

Thanks for your interesting question. The short answer is no, you do not need to use two forms of birth control while on topical tretinoin. The retinoids in general are not recommended in pregnancy, since we don't know how much would be absorbed and potentially affect a fetus, but there are not any cases I know of where a baby was harmed by topical tretinoin use by the mother. You are absolutely right about two forms of birth control being mandatory for oral isotretinoin, but topical tretinoin is not nearly as strong and is not known to cause harm if someone accidentally continues it a little while into a pregnancy. Ask your dermatologist, but you should be fine with just one form of birth control used consistently.