Ensuring and evaluating the safety of vaccines administered to pregnant women is a key component of any maternal immunization program or recommendation. This is particularly true now that new vaccines that can benefit pregnant women and their infants are being developed, such as vaccines to protect against GBS and RSV. An important issue is the need for harmonization of standard definitions of key safety outcomes after maternal vaccination and of a systematic approach to the assessment of safety throughout the life cycle of a vaccine, but particularly after implementation as large number of pregnant women are vaccinated. It is critical to consider the inherent risks associated with pregnancy itself, and to clearly understand the background rate of these risks in specific populations. Furthermore, to evaluate the impact of maternal immunization as a public health strategy to impact the burden of morbidity and mortality associated with the infection it prevents, it is necessary to establish baseline rates of these outcomes to demonstrate the efficacy and benefit of the vaccines in both mothers and infants. Finally, the ethical and regulatory aspects surrounding the inclusion of pregnant women as research subjects also influence the progress of the development of vaccines for maternal immunization.