Background incidence rates of adverse pregnancy outcomes in the Netherlands: Data of 2006-2018.

Abstract Objective Maternal vaccination is an effective and safe intervention to protect newborns against infectious diseases shortly after birth. We assessed background rates of adverse pregnancy outcomes before the implementation of a maternal pertussis immunisation programme in the Netherlands, to put into perspective the safety concerns about such outcomes following immunisation. Study design In this retrospective cross-sectional study, annual numbers of pregnancy outcomes derived from the Dutch Perinatal Registry were used to calculate incidence rates per 10,000 in the 2006-2018 period. Births of ≥500 g birth weight and ≥24 + 0 w gestational age were included. Trends with moving- average-lines over the past 3 years were plotted, with 95% confidence interval. Results From 2006 through 2018, yearly numbers of pregnancies ranged between 158,868-175,710.Numbers of newborns ranged between 161,307-178,874, of whom 160,838-178,177 were live-born. Most outcomes were stable over time. Between 2006-2011, occurrence of labour induction increased by 68%, and postpartum hemorrhage increased by 25%. Both stabilised from 2011 onwards. Perinatal mortality up to day 7 or 28 postpartum decreased by 38% and 37%, respectively. Occurrence of low Apgar score among preterm infants born before 37 + 0 w gestational age and among term infants increased by 19% and 27%, respectively. Conclusion Our study on background incidences showed notable increases over time in occurrence of labour induction, postpartum hemorrhage and low Apgar score, while showing a considerable decrease in overall perinatal mortality. These findings should be considered when interpreting data on adverse events occurring since the maternal pertussis immunisation programme was implemented.