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Background Active inflammatory bowel disease (IBD) in pregnancy may require initiation of corticosteroids or biological therapy to control disease. Data from real world experience is required to understand the impact of these treatments on disease and pregnancy outcomes.Methods A multicentre retrospective cohort study across 16 UK hospitals included pregnant patients with active IBD, defined by symptoms and/or raised inflammatory markers (FCAL ≥250 μg/g or CRP ≥5 mg/L), requiring treatment escalation during pregnancy. Multivariable logistic regression generated adjusted relative risk ratios (aRRR), controlling for maternal age, BMI, smoking status at conception and disease severity, measured by the physician global assessment scale.Results The study included 389 pregnancies. In response to active disease, topical therapy was added in 39.3%, oral 5-ASA in 23.1% (19.5% had dose optimisation). Budesonide was used in 13.9% and systemic corticosteroids (CS) in 33.9%. Thiopurines were started in 3.9% and optimised in 3.1%. A new biologic was commenced in 15.9% (CD 19.7% vs UC/IBD-U 14.1%; p=0.160); Infliximab 44.3%, Adalimumab 27.9%, Vedolizumab 13.1%, Ustekinumab 9.8%, Golimumab 3.3% and Risankizumab 1.6%.The mean [±SD] gestational age at treatment initiation was similar for corticosteroids (CS) and biologics (19.1[8.6] vs 19.4 [8.0] weeks). Where CS (n=90) or biologic initiation (n=30) represented the highest treatment escalation during pregnancy, this was not associated with an increased risk of adverse pregnancy or neonatal outcomes on multivariable analysis.Patients requiring combination CS and biologic therapy (n=32) demonstrated a higher inflammatory burden, with median [IQR] CRP levels (15.1 [7.5–41.0] mg/L, p=0.011) and FCP (1500 [600–2800] μg/g, p=0.109). This was greater than in patients in the CS (CRP 10.0 [5.0–29.0] mg/L; FCP 659 [484–1720] μg/g) or biologics (CRP 9.1 [4.0–18.0] mg/L; FCP 600 [512–2507] μg/g) cohort, consistent with more severe disease activity in the combination therapy group.Combination CS and biologic initiation was associated with higher risk of fetal growth restriction (aRRR 8.85; 95% CI 1.20-65.29, p=0.033), preterm birth (aRRR 3.66, 95% CI 1.03-13.00; p=0.044), emergency c-sections (aRRR 4.02, 95% CI 1.30–12.42; p=0.015) and NICU admission (aRRR 4.79, 95% CI 1.01-22.92; p=0.049).Conclusion Findings from this study support the low risk of antenatal biologic initiation to control disease activity. The higher rate of adverse pregnancy outcomes observed with combined immunosuppression likely reflects the impact of underlying disease severity. This further emphasises the importance of timely escalation of medical therapy to initiate effective therapy to control disease in pregnancy.