The traditional methods for evaluating drug safety in the pre-marketing phases of drug development, i.e. animal reproductive toxicity studies and randomized clinical trials, have limited application with respect to human pregnancy. Owing to species specificity and sensitivity to reproductive toxins, it is difficult to extrapolate with confidence from pre-marketing animal studies to human pregnancy. The species, dose of medication, route of drug administration and maternal toxic effects are some of the experimental variables that can influence findings and limit the interpretability and predictive value of these studies (Brent, 1986).
Clinical trials are the second traditional method of evaluating drug safety. For obvious ethical reasons, pregnant women typically are not recruited for trials during any phase of drug development. If and when unintended pregnancies occur during the course of a trial, pregnancy outcomes can provide preliminary information regarding the risks of exposure. However, these data usually involve very few subjects. There is a trend to include larger numbers of women of childbearing age in clinical trials, and this will undoubtedly result in a larger number of exposed pregnancies in such trials. Nevertheless, these numbers will still be too small to provide meaningful information.
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