Drugs can have harmful effects on the embryo and foetus at any stage (including after birth), yet for some patients denying medication of benefit may led to uncontrolled disease which may be more harmful to both the patient and foetus than the risk of continuing therapy. If you are asked to prescribe for a pregnant woman, you will therefore need to make a careful assessment of the risks and benefits to both the patient and foetus.

Teratogenic effects of drugs

The strict definition of a teratogen is one that causes congenital malformations, however the definition in practice normally includes any agent that causes structural, functional or behavioural abnormalities to the foetus. Assessing the teratogenic risks of medicines is difficult. Ethical issues prevent the inclusion of pregnant patients in randomised controlled trials, hence most data in humans comes from epidemiological studies and case reports. The incidence of major congenital malformations in the general population is between 2-3%, of these only 1-2% are thought to be drug related (UKMi 2012). This means that to show a statistically significant association large studies are required of the order of tens of thousands of patients. For this reason no drug can be considered safe beyond all doubt, but within a class older drugs tend to be used in preference as there will have been more experience. Animal studies can be useful in identifying likely teratogens, but extrapolation to humans cannot be assumed.

Drugs in pregnancy - using the BNF

Look up the following drugs in the BNF. What information is given with regards to use in pregnancy? Warfarin ACE inhibitors Isotretinoin Amoxicillin You may wish to look up other drugs that you are prescribing regularly