Seasonal variation in the incidence of preterm births

To examine the influence of the season of conception, and the season of birth on the incidence of preterm birth (PTB) and neonatal outcomes.

This is a single center, retrospective cohort study of singleton births that took place in The Coombe Hospital in Dublin, Ireland, between January 2013 and December 2022. A comprehensive database was analyzed to determine the incidence of PTB per season of conception and season of birth. Overall neonatal outcomes were reported and stratified per the occurrence of PTB or term birth.

Following exclusions, 76,988 births were analyzed. Women who conceived in winter between December-February had significantly lower rates of PTBs when compared to other seasons (5.4 % vs 6.5 % (spring) vs 5.6 % (summer) and 5.4 % (autumn), p < 0.001). When considering only spontaneous preterm labor, this trend persists, with most women experiencing spontaneous PTBs conceiving during spring (6.7 % vs 5.5 % (winter) vs 5.7 % (summer) vs 5.5 % (autumn), p = 0.001). Conversely, women who gave birth in December-February had significantly higher rates of premature births when compared to other seasons (6.2 vs 5.8, 5.5 %. and 5.5 %, p < 0,01). PTBs of spontaneous onset were highest between December and February, however no statistical significance was found (6.2 % vs 6.1 % (spring), 5.7 % (summer) and 5.4 % (autumn), p = 0.13). No significant variation in maternal risk factors or neonatal outcomes were identified between the seasons.

This study has shown that there is a seasonal variation in the incidence of preterm births in this Irish-based cohort. A low prevalence of PTB was demonstrated when conceptions occurred in the winter months. However, there was a greater incidence of preterm births between December and February. This is suggestive that there are potential risk factors associated with seasonal patterns that may be modifiable. Further research to identify these specific risks is warranted.