Prescribing in pregnancy. Identifying abnormalities.

Drugs taken during pregnancy create concern whether they are self administered or medically prescribed. The number of mothers who take drugs during pregnancy is not known, but a survey in the United States showed that about 45% ofwomen may use at least one drug on prescription, and many more use drugs bought over the counter.' A recent prospective study in the United Kingdom, however, suggested that only about 10% of women took drugs in early pregnancy.2 Defects occur in about 2% to 3% ofbabies at birth, ofwhich about 25% are of genetic origin and 65% are of unknown aetiology. Only 2% to 3% of defects are thought to arise in association with drug treatment.3 The effect of a particular drug on the developing fetus depends on several features, including the type of agent and the gestational age at which it was taken. The aim of this article is to provide a guide for doctors treating pregnant women who have taken, or who are currently taking, drugs. Advice depends on the following: (a) knowledge of the timing of embryonic and fetal development because preparations taken outside the critical phases are unlikely to be teratogenic; (b) the precise nature of the drug so that the teratogenic effect may be determined (table); and (c) whether the teratogenic effect is likely to be sufficiently apparent to make prenatal diagnosis feasible.