Objective This study aimed to investigate the therapeutic effect, mechanisms, and safety of hypoxia-pretreated mesenchymal stem cell-derived exosomes ( hMSC-Exos) in a rat model of gestational diabetes mellitus (GDM), and evaluate their potential as a novel therapeutic strategy. Methods Human umbilical cord mesenchymal stem cell were cultured under hypoxic conditions ( 5% O2 ), and exosomes were isolated with ultrafiltration and ultracentrifugation. Female SD rats were randomly divided into 4 groups: normal pregnancy control (Control), GDM model (Diabetic), hMSC-Exos treatment (Exosome), and insulin treatment (Insulin) groups (n= 10 per group). The GDM model was established by intraperitoneal injection of streptozotocin ( STZ, 1%, pH =4. 5) on gestational day ( GD) 0. Body mass and blood glucose levels were measured daily. Hemoglobin A1c (HbA1c) and homeostatic model assessment of insulin resistance (HOMA-IR) were evaluated on GDs 0, 5, 10, and 15. Area under the curve of the oral glucose tolerance test (OGTT-AUC) was calculated after a 12-hour fast on GD0 and 15. Fetal development was monitored by ultrasound at different stages. At the end of pregnancy, complete blood count, lymphocyte subsets T helper 17 cell ( Th17) and regulatory T cell ( Treg) and pregnancy outcomes were analyzed. Results Compared with the Diabetic group, the Exosome group exhibited significantly increased body mass from GD15 (P<0. 0001), and significantly reduced blood glucose from GD10 (P<0. 0001), normalizing by GD20. HbA1c decreased significantly in the Exosome group from GD 10 (P<0. 0001), showing a 22. 6% reduction at GD15. At term, HOMA-IR was significantly lower in the Exosome group (P<0. 0001). Post-treatment (GD15), OGTT-AUC in the Exosome group decreased by 46. 65% (P<0. 0001). Ultrasound indicated improved fetal growth in the Exosome group. Treatment caused no severe hematotoxicity or immune dysregulation; Th17 proportion was significantly lower (P<0. 01), Treg proportion significantly higher (P<0. 05), and the Th17 / Treg ratio decreased 3-fold in the Exosome group compared with the findings in the Diabetic group. Fetal mass and number were significantly higher in the Exosome group (P<0. 001). Conclusions hMSC-Exos therapy effectively ameliorates glucose metabolism, alleviates insulin resistance, modulates Th17 / Treg immune balance to improve the gestational milieu, and promotes fetal development in GDM rats, demonstrating favorable safety.