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Pharmaceutical Association of Mauritius



 Safety during pregnancy

A teratogen is an agent which causes structural or functional abnormalities in the fetus, or in the child after birth. In the UK, the proportion of spontaneous abortions in clinically recognised pregnancies is 10-20% and of gross malformations is estimated to be about 3%.1 The cause of most malformations is not known but at least 2-4% are due to drugs or chemicals.1

Known teratogenic drugs

The well-known teratogenic effects of thalidomide provided the main stimulus for the introduction of modern drug regulation, including the Yellow Card Scheme. Other commonly recognised teratogenic drugs include androgens, cytotoxic agents, lithium, retinoids, and warfarin. Drugs should only be prescribed in pregnancy if the benefits for both mother and unborn child outweigh the risks. For example, in women with a history of epilepsy, prescription of potentially teratogenic anticonvulsants is often required to prevent seizures, which may be associated with hypoxic CNS damage to the fetus, or intra-uterine death.2 Appendix 4 of the British National Formulary provides a valuable source of information on drugs and pregnancy.

Detecting potential teratogens

During development, drugs undergo studies in animals to assess their potential as teratogens. However, lack of a teratogenic effect in animals does not guarantee safety in human pregnancy. Once a drug is marketed, the Yellow Card Scheme is an important method for generating signals which then can be more formally investigated. A further data collection system in the UK is the National Teratology Information Service.3 This service follows up enquiries regarding patients who have received newly introduced drugs, known or suspected teratogens, or who have been exposed to occupational and environmental chemicals while pregnant, to obtain data on pregnancy outcome.

Assessing causality

Confirming that a drug is a teratogen may be difficult. Epidemiological studies can provide quantitative estimates of the strength and statistical significance of associations between drug exposure in pregnant women and congenital abnormalities. Such studies were used to confirm the association between pre-natal exposure to diethylstilboestrol and vaginal and cervical abnormalities including vaginal adenocarcinoma in female offspring.4 Epidemiological studies have several limitations. For example, the maternal disease requiring drug treatment may itself have resulted in the observed association. Spurious associations can occur or important risks may be missed in investigations involving small numbers of affected patients. Furthermore, women who have had a child with a birth defect are more likely to remember the drugs taken during pregnancy than women who have had a normal child. Assessment of the teratogenicity of a drug must be made, therefore, on the basis of the reproducibility, consistency and biological plausibility of the combined experimental, clinical and epidemiological data.

Important links

Pregnancy and Lactation - From Harbor UCLA Medical center - lists drugs' pregnancy category and extent of excretion into breast milk.