A teratogen is any substance, organism, or physical agent that can disturb an embryo’s or foetus’s normal development, causing birth defects. Well-known teratogens include alcohol, certain medicines such as thalidomide and some acne drugs, infections such as rubella, and high doses of vitamin A in its retinol form — which is why pregnant women are advised against liver and retinol supplements. The risk depends on dose and timing: the embryo is most vulnerable during the first trimester, when organs are forming. In food safety, teratogenicity is one of the endpoints tested before additives, pesticides, and contaminants are approved, and exposure limits are set to protect the unborn.