The fetal safety of the ondansetron was first investigated in humans by Einarson et al7 in 2004 through a prospective controlled cohort study of 176 women, in whom we could not detect an increased teratogenic risk. However, this sample size had the statistical power to rule out only a 5-fold increased risk of major malformations, and not any specific malformation. In February 2013, Pasternak et al8 reported that ondansetron was not associated with increased malformation rates when used for morning sickness. This was based on retrospective analysis of data from the Danish Birth Registry, collected between 2004 and 2011 and linked to the National Prescription Register. Each of the 1970 women exposed to ondansetron was matched to 4 unexposed controls. Of note, the mean age at exposure was 10 gestational weeks, which means that half of the cases were exposed to ondansetron at later than 10 gestational weeks, when the morphologic malformations sought in this study could not be produced any more. This can cause a bias toward the null, diluting an existing risk because of inclusion of cases that were not exposed during embryogenesis.