Drugs can have harmful effects (teratogenic effects) on the foetus at any time during pregnancy, with the period of greatest risk being from the third to the eleventh week of pregnancy.
The safety, efficacy and quality of each new medicine is thoroughly evaluated as part of the licensing procedure. European guidelines define the types of tests which should be undertaken, including those that assess the potential harm to a developing foetus.
The available information is reflected in the product information for each medicine which contains a specific section that summarises the information in relation to safety of use during pregnancy. This is kept under review to ensure patients and prescribers are given accurate up to date information. A summary source of this information is appendix 4 of the British National Formulary (BNF). The BNF is provided free to prescribers and pharmacists within the NHS.