Abstract Background Corticosteroids, thiopurines, and biologics may come into play during pregnancy in women with inflammatory bowel disease and potentially impact the developing fetal immune system. We aimed to assess the risk of serious infections in children stratified by in utero exposure to biologics and immunomodulators or concomitant treatment with corticosteroids. Methods All singleton IBD pregnancies between 2008 and 2022 at a tertiary IBD center in Denmark were included. Maternal and offspring demographics, maternal disease activity, antenatal medical treatment, and infant infections resulting in hospital admission were recorded after review of medical records. Results In 602 live births (99.0%), we registered exposure to antenatal treatment as follows: biological monotherapy (n = 61, 10.2%), thiopurines (n = 110, 17.9%), biologics and concomitant thiopurines (n = 63, 10.3%), and controls (ie, no treatment with biological and/or thiopurines; n = 369, 60.6%). Preterm delivery (<37 gestational weeks) and systemic steroid administration during the third trimester were associated with an increased risk of serious infection in the offspring immediately after birth (relative risk = 17.5; 95% confidence interval, 7.8-39.8; P < .001, and relative risk = 4.8; 95% confidence interval, 1.5-12.7; P = 0.003, respectively). Intra-uterine exposure to biologics or combination treatment were not associated with a statistically significant higher risk of serious infections compared with controls; however, combination treatment showed an inclination towards an increased risk across analyses. Conclusion Preterm birth and systemic corticosteroid administration late in pregnancy are significant risk factors for serious infections in the offspring of IBD mothers.