Background
Inflammatory bowel disease (IBD) and infertility have each been associated with adverse pregnancy outcomes, including preterm birth (PTB). However, the joint effects of IBD and infertility on PTB remain unclear.

Methods
A retrospective, population‑based cohort of women aged 15-55 years with a first recorded delivery (≥ 20 weeks’ gestation), in Ontario, Canada (2007–2024) was constructed using individual-level health administrative datasets, including MOMBABY (mother-baby linkage) and BORN (perinatal registry). Exposure was defined as a four‑level variable: 1) IBD alone; 2) infertility diagnosis and/or treatment alone; 3) joint exposure; 4) neither (comparator). Modified overlap‑weighted Poisson models were used to address confounding by indication. Interaction was quantified using the Relative Excess Risk due to Interaction (RERI) and Attributable Proportion (AP).

Results
Among 862,515 women with a first recorded delivery, 4761 had IBD, 122,340 had infertility, and 1276 had both IBD and infertility. Overall, 7.3% of women experienced a PTB, with a higher proportion among women with IBD and infertility (14.7%), with infertility (11.4%), and with IBD (9.7%). Compared with women with neither IBD nor infertility, the risk of PTB was 79% higher among women with both IBD and infertility (RR 1.79, 95% CI 0.92-2.10), 45% higher for women with infertility alone (RR 1.45, 95% CI 1.42-1.48), and 9% higher among women with IBD alone (RR 1.09, 95% CI 0.98-1.21). Joint exposure was found to be associated with a 25% excess risk of PTB, with 14% of the risk of PTB attributable to joint exposure.

Conclusion
IBD and infertility exert a synergistic effect on the risk of PTB. These findings underscore the importance of integrated preconception and prenatal care for individuals experiencing both IBD and infertility and highlight opportunities for risk stratification to improve their clinical management of pregnant women living with these conditions.