This retrospective pregnancy case was reported in a literature article and described the occurrence of hypertrophic pyloric stenosis in a neonate subject of unspecified gender whose mother was vaccinated with H1N1 (manufacturer unspecified) during pregnancy. On an unspecified date, the mother of the subject received an unspecified dose of H1N1 (transplacentary, unknown injection site, batch number not provided). The mother's last menstrual period was on unspecified date and estimated date of delivery was not provided. The neonate was exposed to the vaccine during the first trimester. On unspecified date, at unspecified weeks gestation, the neonate was delivered by unknown delivery method (birth weight and APGAR score(s) were not provided). Birth defects were noted. The neonate was born with hypertrophic pyloric stenosis. The author considered the events were a congenital anomaly. At the time of reporting, the outcome of the event was unspecified. The authors reported 33 other cases. Summary of the article: The potential serious consequences of maternal influenza infection in pregnancy have long been recognized; pregnant women are at increased risk for influenza-associated morbidity and mortality. As reviewed recently, there are limited data on the safety of influenza vaccine in pregnancy; however, to date, there has been no indication that seasonal influenza vaccines containing inactivated virus pose a risk to the mother or the foetus. This has led to the current recommendation that all pregnant women be vaccinated with the seasonal influenza vaccine regardless of trimester. Despite these recommendations, vaccination rates remain less than optimal. Surveys have demonstrated that mothers' concerns about safety contribute to poor compliance with vaccine recommendations. This situation was escalated during the 2009-10 H1N1 pandemic season, when lack of safety data about the pandemic H1N1 (pH1N1) monovalent vaccine was coupled with intensive public health messaging that emphasized the need for all pregnant women to be vaccinated. Additional safety information on pH1N1-containing vaccines when used in pregnancy was therefore needed. The purpose of this study was to evaluate the foetal risk and relative safety of the pH1N1 influenza vaccine. To assess risks and relative safety of the pH1N1-containing vaccines, the author conducted a prospective cohort study of pH1N1-vaccine-exposed and unexposed comparison women who were recruited during pregnancy and followed to outcome between October 2009 and August 2012. There were 1032 subjects available for analysis; 841 women were exposed to a pH1N1-containing vaccine in pregnancy, and 191 women were unexposed to any influenza vaccine in pregnancy. Nine of 328 (2.7%) first-trimester-exposed pregnancies resulted in an infant with a major birth defect compared to 6/188 (3.2%) in the unexposed. The hazard ratio (HR) for spontaneous abortion was not elevated. Adjusted HRs for preterm delivery were elevated for exposure anytime in pregnancy, specifically with exposure in the 1st or 2nd trimester. However, the mean decrease in gestational age in the exposed pregnancies was approximately three days. In conclusions, no meaningful evidence of increased relative risk or HR for major birth defects, spontaneous abortion, or small for gestational age infants was demonstrated.