LAUSR

Preterm birth is a substantial global health issue with well-documented consequences for the infant, family, and society at large. Identified as the leading cause of death among children younger than 5 years, preterm delivery occurs in approximately 15 million births worldwide and remains one of the foremost issues in the field of obstetrics. 1 Preterm birth rates are increasing internationally. 1 In 2019, the most recent year with complete data available, the rate of preterm birth in the US was 10.23%, and the rate of births earlier than 28 weeks’ gestation was 0.66%. 2 Gestational age at delivery and the risk of neonatal morbidity and mortality are inversely associated. Neonates born earlier than 28 weeks’ gestation comprise the smallest proportion of births, but these infants experience disproportionately higher rates of prematurity-related complications, including death. 3 For context, the infant mortality rate among those born earlier than 28 weeks’ gestation was 186 times higher than those born between 37 and 41 weeks’ gestation in the US in 2018. 3 Infants who survive are at risk of developing a wide range of short- and long-term morbidities. Although the burden of preterm birth is clear, understanding the biological characteristics of human parturition remains elusive. To this end, modifiable risk factors are being explored to address this substantial public health issue. Hviid and colleagues 4 report data from a population-based retrospective cohort study in Denmark examining the association of birth season and pregnancy onset season with rates of preterm birth. Seasons were defined as winter (December, January, and February), spring (March, April, and May), summer (June, July, and August), and autumn (September, October, and November). Preterm births were categorized according to severity, with extremely preterm defined as live births occurring between 22 and 27 weeks’ gestation, very preterm as live births occurring between 28 and 31 weeks’ gestation, and moderately preterm as live births occurring between 32 and 36 weeks’ gestation. Because of the substantial burden of extreme