Objective To assess background rates of adverse pregnancy outcomes before implementation of a maternal pertussis immunisation programme in the Netherlands, to put into perspective the safety concerns about such outcomes following immunisation. Design Retrospective cross-sectional study. Setting and population Dutch pregnant women and their infants who were registered in the Dutch Perinatal Registry in the 2006-2018 period. Births of ≥500g birth weight and ≥24+0w gestational age were included. Methods Annual numbers of adverse outcomes were used to calculate incidence rates per 10,000. Trends with moving-average-lines over the past 3 years were plotted, with 95% confidence interval. Main Outcome Measures Adverse outcomes at maternal and neonatal level. 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+0w gestational age and among term infants increased by 19% and 27%, respectively. Conclusions 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 Tdap vaccination was implemented.