Perinatal & Pediatric
Time-related bias when studying perinatal complications after maternal injuries: Application to maternal injuries and preterm birth Asma M. Ahmed* Asma Ahmed Allison Musty Joseph Rigdon Jennifer A. Hutcheon
Background
Injuries affect 1 in 12 pregnancies and are associated with perinatal complications. Some studies that examined associations between maternal injuries and preterm birth (PTB) reported null or counterintuitive protective effects, especially for 3rd-trimester injuries, likely due to time-related biases (Figure). We estimated associations between maternal injuries and PTB while appropriately accounting for time-related biases.
Methods
A retrospective cohort study including all births from Atrium Health Wake Forest Baptist system in 2018-2024. Maternal injuries were ascertained using validated diagnostic codes. PTB was defined as gestational age at delivery <37 weeks. We estimated associations between maternal injuries and PTB via two approaches. We used logistic regression for time-fixed analysis (i.e., injury at any point in pregnancy yes/no and PTB yes/no) and Cox proportional hazards models for time-varying analysis (i.e., time-varying injury exposure definition, restricting follow-up to periods when pregnancies were at risk of PTB).
Results
Among 58897 births, 3.1% experienced maternal injuries, and 11.2% were PTB (12.4% among injured and 11.2% among uninjured). With the time-varying approach, maternal injuries were associated with increased risk of PTB (adjusted HR: 1.16 (95% CI: 1.01-1.32)). Trimester-specific analyses showed positive associations for all trimesters, with higher effect estimates for 2nd and 3rd trimester injuries (HR 1.17 (0.97-1.42) and 1.22 (0.92-1.61), respectively). With time-fixed analyses, associations were underestimated, and results for 3rd trimester injuries showed counterintuitive negative associations (HR: 0.74 (0.56-0.97)).
Conclusion
Time-related biases typically underestimate associations between maternal injuries and preterm birth, particularly for 3rd trimester injuries. Rigorous study design and analytical methods that account for time-related biases are crucial in studies investigating adverse outcomes after maternal injury.
