Approximately 1% of congenital anomalies relate to pharmacological exposure and are. in theory, preventable. Prevention consists of controlled administration of drugs known to have teratogenic properties (e.g. retinoids, thalidomide). When possible, prevention could take the form of the use of alternative pharmacological therapies during the pre-conception period for certain specific pathologies, selecting the most appropriate agent for use during pregnancy [e.g. haloperidol or a tricyclic antidepressant instead of lithium; anticonvulsant drug monotherapy in place of multitherapy; propylthiouracil instead of thiamazole (methimazole)], and substitution with the most suitable therapy during pregnancy (e.g. insulin in place of oral antidiabetics; heparin in place of oral anticoagulants; alpha-methyldopa instead of ACE inhibitors). Another strategy is the administration of drugs during pregnancy taking into account the pharmacological effects in relation to the gestation period (e.g. avoidance of chemotherapy during the first trimester, avoidance of nonsteroidal anti-inflammatory drugs in the third trimester, and avoidance of high doses of benzodiazepines in the period imminent to prepartum).