Objective: In utero exposure to maternal inflammation may impact immune system development and subsequent risk of disease. We investigated whether a maternal diagnosis of IBD before childbirth is linked to a higher risk of IBD in offspring compared with a diagnosis after childbirth. Further, we analysed paternal IBD status for comparison., Design: Using Danish health registers, we identified all individuals born in Denmark between 1997 and 2022 and their legal parents, as well as their IBD status. Cox proportional hazards regression analyses adjusted for calendar period and mode of delivery were used to estimate offspring IBD risk by maternal and paternal IBD status before and after childbirth., Results: Of 1 290 358 children, 10 041 (0.8%) had mothers with IBD diagnosis before childbirth and 9985 (0.8%) had mothers with IBD diagnosis after childbirth. Over 18 370 420 person-years, 3537 individuals were diagnosed with IBD. Offspring of mothers with IBD before childbirth had an adjusted HR of IBD of 6.27 (95% CI 5.21, 7.54) compared with those without maternal IBD, while offspring of mothers with IBD after childbirth had an adjusted HR of 3.88 (95% CI 3.27, 4.60). Corresponding adjusted HRs were 5.26 (95% CI 4.22, 6.56) among offspring with paternal IBD before childbirth and 3.73 (95% CI 3.10, 4.50) for paternal IBD after childbirth., Conclusion: Offspring had a greater risk of IBD when either parent was diagnosed before childbirth rather than later, emphasising genetic predisposition and environmental risk factors rather than maternal inflammation in utero as risk factors for IBD., Competing Interests: Competing interests: The corresponding author confirms on behalf of all authors that there have been no involvements that might raise the question of bias in the work reported or in the conclusions, implications, or opinions stated. The authors disclose the following: MA is supported by the National Institute of Diabetes and Digestive and Kidney Diseases (K23DK129762-03). MA reports consulting for Douglas Pharmaceutical. MJ has received research grants for other investigator-driven studies from Takeda, and the NOVO Nordisk Foundation (grant no. NNF23OC0081717), has been on the advisory board of Pharmacosmos, has received consultation fee from Ferring and Takeda, and has received speaker’s fees from Tillotts Pharma, MSD, Ferring and Takeda. JT has received grant support from Abbvie and Janssen, consulting fees from Janssen, Abbvie and Pfizer, and speaker fees from Janssen, Abbvie, Pfizer and Sandoz. TJ reports having consulted for Ferring and Pfizer. The other authors disclose no conflicts., (© Author(s) (or their employer(s)) 2024. No commercial re-use. See rights and permissions. Published by BMJ.)