Retinoids are the one skincare ingredient that is definitively unsafe during pregnancy. Oral isotretinoin (Accutane) is a known teratogen that causes severe birth defects including craniofacial, cardiac, and central nervous system abnormalities. It must be discontinued at least one month before attempting conception.
Topical retinoids (retinol, tretinoin, adapalene, tazarotene) carry a much lower risk because skin absorption is limited. However, they are still contraindicated during pregnancy because the theoretical risk exists and effective alternatives are available. There is no reason to use a retinoid when vitamin C and azelaic acid can address the same concerns safely.
If you were using a retinoid and discovered you were pregnant, stop it immediately and tell your provider. Brief topical retinoid use in early pregnancy is not associated with the birth defect pattern seen with oral isotretinoin, but discontinuation is still recommended.