OBJECTIVE: Prior research efforts have not effectively clarified the relationship between preconception body mass index (BMI) and spontaneous preterm birth among women with gestational diabetes mellitus (GDM), particularly among Asian women. This study explores the relationship between pre-pregnancy BMI and spontaneous preterm birth among women with GDM, taking into account triacylglycerol (TG), glycated hemoglobin A1c (HbA1c), and gestational weight gain (GWG) levels. METHOD: Data from 1116 women with GDM who produced singleton live births were retrospectively analyzed. Logistic regression models were used to explore the relationship between pre-pregnancy BMI and spontaneous preterm birth. RESULTS: Relative to women with GDM with a BMI in the normal range, those who were overweight/obese exhibited a significantly higher risk of spontaneous preterm birth. This link between BMI and spontaneous preterm birth risk varied as a function of GWG, HbA1c, and TG. Specifically, among those women who were overweight/obese prior to pregnancy, TG levels were significantly related to spontaneous preterm birth risk at TG concentrations exceeding 1.69 mmol/L. Notably, the combination of overweight/obese status, excessive GWG, and/or HbA1c levels ≥5.5% was associated with a higher risk of spontaneous preterm birth relative to overweight/obese status alone. CONCLUSION: These data show that preconception BMI is significantly linked to the risk of spontaneous preterm birth among Chinese GDM patients. When examining this relationship, HbA1c, TG, and GWG levels should be taken into consideration.