Sources of bias in signals of pharmaceutical safety in pregnancy

Every year scores of new pharmaceuticals enter the market, almost never with human fetal safety data. Such data typically accumulate during the first years of clinical use, in the form of case reports, case series, prospective and retrospective cohorts and case control studies. All of these methods suffer from serious sources of bias, often leading to alarming signals of teratogenicity that are later found to be false. This review highlights major sources of bias, including the bias against the null hypothesis in its different forms, ascertainment and recall bias, in fetal exposure to pharmaceutical molecules.