OBJECTIVES: Previous data on inflammatory bowel disease (IBD) relapse during pregnancy mainly originate from retrospective studies. The aim of this study was therefore (i) to evaluate the effect of active disease at conception and IBD disease type on disease relapse during pregnancy and (ii) to study the effects of disease relapse during pregnancy on birth outcomes in a prospective cohort with adequate representation of current treatments. METHODS: From 2008 to 2014, IBD women were recruited from an ongoing prospective clinical cohort. All patients with confirmed IBD diagnosis with a pregnancy wish or pregnancy were prospectively followed-up until pregnancy and delivery. Disease relapse was measured at each visit. Birth outcomes were recorded from the obstetrician. RESULTS: A total of 298 pregnancies were observed in 229 IBD patients (157 Crohn's disease (CD), 66 ulcerative colitis (UC), and 6 IBD unclassified), resulting in 226 live births. Active disease at conception was strongly associated with disease relapse during pregnancy (aOR=7.66, 95% confidence interval (CI): 3.77-15.54). UC patients experienced relapse during pregnancy more often than CD patients, independent of maternal age, smoking, periconceptional disease activity, previous IBD surgery, and the use of immunosuppressives or anti-tumor necrosis factor (TNF) (aOR=3.71, 95% CI: 1.86-7.40). Disease relapse was not associated with adverse birth outcomes such as spontaneous abortion, low-birth weight, or preterm birth. CONCLUSIONS: This study confirms that active disease around conception increases the risk of disease relapse during pregnancy. In addition, UC patients relapse more often during pregnancy than CD patients. Birth outcomes were excellent, reflecting the stringent follow-up and treatment of this group of patients.