Seasonal variation in early preterm birth across Robson groups: A five-year cohort study

Abstract

Objective

To investigate the association between seasonal patterns in births according to the Robson classification and early preterm birth rates.

Methods

In this retrospective cohort study, 13,889 births at a tertiary referral hospital between September 2019 and September 2024 were analyzed. Robson groups were assigned based on obstetric characteristics. Early preterm birth (<32 weeks) was identified using recorded gestational age. Seasonal distribution was defined by date of birth. Associations were assessed using chi-square tests and multivariable logistic regression adjusted for Robson group, season, and maternal age.

Results

Among 13,889 births, 1472 (10.6%) were preterm (<37 weeks), including 120 early preterm births (<32 weeks; 0.86% of all births). Early preterm birth risk differed significantly across Robson groups (p < 0.001). In multivariable analysis, Robson Group 8 (OR 10.36, 95% CI 4.33–24.78), Group 9 (OR 11.60, 95% CI 3.32–40.51), and Group 10 (OR 5.51, 95% CI 2.52–12.04) were independently associated with increased early preterm birth risk compared with Group 1. Although descriptive seasonal differences were observed, season was not an independent predictor after adjustment.

Conclusion

Early preterm birth risk varies substantially across specific Robson groups. While seasonal differences were observed descriptively, seasonality did not independently predict early preterm birth. These findings support interpreting season as a contextual modifier rather than a direct causal determinant and underscore the need for multicenter studies incorporating trimester-specific exposure assessment.