Congenital malformations can be defined as nonreversible functional or morphological defects present at birth. Of course, not all malformations can be attributed to drug use during gestation. Environmental factors alone, such as maternal disease or infection, chemicals and drugs, account for between 5% and 9% of the malformations. Only a small portion of these are due to drugs acting as teratogens. However, this is still a very significant number to the one or two families out of every 1,000 with a child born with a major congenital malformation.
The risk of teratogenicity must be described in relative terms, since modern science has no way to predict with any real accuracy which drug exposures will result in teratogenesis during a particular pregnancy. Available data allow only the description of a relative risk to a patient population. Although there are data describing the effects of many drugs on animal development, these effects are not uniformly applicable to human pregnancies.