OBJECTIVE: To investigate causes of discrepancies in the association between early pregnancy oral glucocorticoid (OGC) use and preterm birth risk among women with rheumatoid arthritis (RA) in health care utilization data from California Medicaid (Medi-Cal) and the prospective cohort MotherToBaby Pregnancy Studies. METHODS: Separately, we estimated risk ratios (RRs) between OGC exposure before gestational day 140 and preterm birth risk in data from Medi-Cal (2007-2013; n = 844) and MotherToBaby (2003-2014; n = 528). We explored differences in socioeconomic status, OGC dose distribution, exposure misclassification, and confounding by RA severity across the data sources. RESULTS: Preterm birth risk in women without OGC was 17.3% in Medi-Cal and was 9.7% in MotherToBaby. There was no association between OGC and preterm birth in Medi-Cal (adjusted RR 1.00 [95% confidence interval (95% CI) 0.71, 1.42]), and a 1.85-fold (95% CI 1.20, 2.84) increased preterm birth risk in MotherToBaby. When restricting each sample to women with a high-school diploma or less, preterm birth risk following no OGC exposure was 15.9% in Medi-Cal and 16.7% in MotherToBaby; adjusted RRs were 1.16 (95% CI 0.74, 1.80) in Medi-Cal and 0.81 (95% CI 0.25, 2.64) in MotherToBaby. Cumulative OGC dose was higher in MotherToBaby (median 684 mg) than in Medi-Cal (median 300 mg). An OGC dose of ≤ 300 mg was not associated with increased preterm birth risk. Exposure misclassification and confounding by RA severity were unlikely explanations of differences. CONCLUSION: Higher baseline preterm birth risk and lower OGC dose distribution in Medi-Cal may explain the discrepancies. Studies are needed to understand the effects of autoimmune disease severity and undertreatment on preterm birth risk in low-income populations.