BACKGROUND: There is limited information regarding the impact of dose and gestational timing of oral corticosteroid (OCS) use on preterm birth (PTB), especially among women with asthma. OBJECTIVES: To evaluate OCS dose and timing on PTB for asthma and, as a comparison, systemic lupus erythematosus (SLE). METHODS: We used health care data from California Medicaid enrollees linked to birth certificates (2007-2013), identifying women with asthma (n = 22,084) and SLE (n = 1174). We estimated risk ratios (RR) for OCS cumulative dose trajectories and other disease-related medications before gestational day 140 and hazard ratios (HR) for time-varying exposures after day 139. RESULTS: For asthma, PTB risk was 14.0% for no OCS exposure and 14.3%, 16.8%, 20.5%, and 32.7% in low, medium, medium-high, and high cumulative dose trajectory groups, respectively, during the first 139 days. The high-dose group remained associated with PTB after adjustment (adjusted RR [aRR]: 1.46; 95% confidence interval [CI]: 1.00, 2.15). OCS dose after day 139 was not clearly associated with PTB, nor were controller medications. For SLE, PTB risk for no OCS exposure was 24.9%, and it was 39.1% in low- and 61.2% in high-dose trajectory groups. aRR were 1.80 (95% CI: 1.34, 2.40) for high and 1.24 (95% CI: 0.97, 1.58) for low groups. Only prednisone equivalent dose >20 mg/day after day 139 was associated with increased PTB (adjusted HR: 2.54; 95% CI: 1.60, 4.03). CONCLUSIONS: For asthma, higher OCS doses early in pregnancy, but not later, were associated with increased PTB. For SLE, higher doses early and later in pregnancy were associated with PTB.