INTRODUCTION Increased C-reactive protein (CRP) concentrations during pregnancy are associated with several perinatal complications. OBJECTIVES The aim of the study was to assess serum CRP concentrations and identify its determinants in pregnant women with type 1 diabetes. PATIENTS AND METHODS CRP concentrations were determined using a high-sensitivity assay (hs-CRP) in the first trimester (I, week <12 of gestation), in mid-pregnancy (II, weeks 20 to 24 of gestation), and in the late third trimester (III, weeks 34 to 39 of gestation) in a group of 73 patients with type 1 diabetes. RESULTS There was a significant increase in CRP concentrations between the first trimester and mid‑pregnancy (median [interquartile range], 2.5 mg/l [1.3-4.5 mg/l] and 5.6 mg/l [2.5-11.6 mg/l]; P = 0.0001), which then stabilized with no further change between mid-pregnancy and the late third trimester (5.7 mg/l [2.5-9.6 mg/l]). CRP concentrations in all 3 trimesters were positively correlated with the waist‑to-hip ratio (I, P <0.0001; II, P = 0.0004; III, P = 0.0369) and body mass index (I, P = 0.015; II, P = 0.0025; III, P = 0.0048), measured in the first trimester. CRP concentrations during pregnancy were positively correlated with a measure of insulin resistance, namely, the estimated glucose disposal rate, assessed in the first trimester (I, P = 0.01; II, P = 0.0165; III, P = 0.0062). There was a positive correlation between the levels of hs-CRP and total cholesterol (P = 0.001), low-density lipoprotein cholesterol (P = 0.013), and triglycerides (P = 0.0014) in the first trimester. There was no significant correlation between CRP and hemoglobin A1c, daily insulin requirement/kg, high-density lipoprotein cholesterol levels, maternal age, and diabetes duration. CONCLUSIONS Adiposity, abnormal body fat distribution, and insulin resistance are the major determinants of CRP concentrations in pregnant women with type 1 diabetes. Our results confirm the importance of weight control before pregnancy in women with type 1 diabetes.