Vaccine-preventable infectious diseases are responsible for maternal, morbidity and mortality. Immunization of pregnant women can protect against vaccine-preventable infections, and may have the added benefit of direct fetal protection. Outcomes of spontaneous abortion and ectopic pregnancy following maternal receipt of vaccination have been less studied. There have been few prospective clinical trials evaluating vaccination in pregnancy; most safety data available are derived from registries where outcomes are passively reported. Spontaneous abortion and ectopic pregnancy are important pregnancy outcomes that should be included in vaccine registries or included as important outcomes in vaccine research. As many organizations define pregnancy loss uniquely we will compare and contrast the existing definitions and provide guidance for use of this adverse event term in studies of maternal immunization.