Background Maternal type 1 diabetes (T1D) has been linked to preterm birth and other adverse pregnancy outcomes. How these risks vary with glycated hemoglobin (or hemoglobin A1c [HbA1c]) levels is unclear. Objective To examine preterm birth risk according to periconceptional HbA1c levels in women with T1D. Design Population-based cohort study. Setting Sweden, 2003 to 2014. Patients 2474 singletons born to women with T1D and 1 165 216 reference infants born to women without diabetes. Measurements Risk for preterm birth (