Preterm birth after the introduction of COVID-19 mitigation measures in Norway, Sweden and Denmark: a registry-based difference-in-differences study

Background Although some studies have reported a decrease in preterm birth following the start of the COVID-19 pandemic, findings are inconsistent. Objective This study aimed to compare the incidence of preterm birth before and after the introduction of COVID-19 mitigation measures in Scandinavian countries, using robust population-based registry data. Study design Registry based difference-in-differences study using births from January 2014 through December 2020 in Norway, Sweden and Denmark. Changes in preterm birth (<37 weeks) rates before and after introduction of COVID-19 mitigation measures (set to March 12, 2020) were compared to changes in preterm birth before and after March 12 in 2014-2019. Differences per 1000 births were calculated for 2, 4, 8, 12 and 16 week intervals before and after March 12. Secondary analyses included medically indicated preterm birth, spontaneous preterm birth, and very preterm (<32 weeks) birth. Results 1,519,521 births were included in this study. During the study period 5.6% of births were preterm in Norway and Sweden, and 5.7% in Denmark. There was a seasonal variation in the incidence of preterm birth, with highest incidence during winter. In all three countries, there was a slight overall decline in preterm births from 2014 to 2020. There was no consistent evidence of a change in preterm birth rates following the introduction of COVID-19 mitigation measures, with DiD estimates ranging from 3.7/1000 births (95% CI -3.8 to 11.1) for the first two weeks after March 12, 2020, to -1.8/1000 births (95% CI -4.6 to 1.1) in the 16 weeks after March 12, 2020. Similarly, there was no evidence of an impact on medically indicated preterm birth, spontaneous preterm birth, or very preterm birth. Conclusions Using high quality national data on births in three Scandinavian countries, each of which implemented different approaches to address the pandemic, there was no evidence of a decline in preterm births following the introduction of COVID-19 mitigation measures.