Every drug has the potential to affect the fetus but few are definitely teratogenic. The prescription or inadvertent use of drugs in pregnancy often causes considerable anxiety in parents because of their concerns about fetal harm. Accurate information and appropriate counselling are required to avoid unnecessary terminations and improve patient compliance with essential treatment. This article outlines the principles of teratogenicity and gives practical advice regarding prescribing in pregnancy. The importance of pre-pregnancy counselling and stabilizing medical disorders in pregnancy is emphasized. Changes in maternal physiology that can affect pharmacokinetics and factors influencing transplacental passage of drugs are also addressed.
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