Prescribing in pregnancy and during breast feeding: using principles in clinical practice.

Prescribing in pregnancy often causes uncertainty and anxiety for the clinician and may lead to the omission of necessary treatment. Many drugs have inadequate data to assure safety, and therefore the clinician is left with a dilemma as to where the balance of risks and benefits lie with respect to the mother and her fetus. Understanding under what circumstances women can be prescribed medication and using principles of prescribing in pregnancy to further clarify the potential risks will aid good clinical decision-making. An appreciation of the available resources and the conviction to find the best available evidence will best serve the patient and her fetus. Teratogenicity refers to the potential for a drug to cause fetal malformations and affects the embryo 3-8 weeks after conception. Teratogenic drugs are associated with an increased risk of malformations, but the majority of babies are born with no abnormalities. In addition, approximately 2-3% of infants are born with major malformations with no association with maternal medication, and this and other confounding factors need to be addressed during counselling of a woman. Fetotoxicity refers to the functional changes that can occur to the fetus as a result of medication in the second and third trimesters. These effects are more subtle and more difficult to assess and therefore there are fewer data to support or refute these types of associations. For the majority of drugs, the neonatal dose from breast feeding is a fraction of the dose exposure in utero.