Background: ABO and Rhesus blood systems are associated with type 2 Diabetes Mellitus (DM2). Gestational Diabetes (GDM) is a model to study DM. Aim: To study the association between GDM and ABO and Rhesus groups. Material and methods: A retrospective cohort study was performed in 1078 women who gave birth to a singleton in Talca Regional Hospital, Chile, during 2008. We analyzed personal, obstetric, medical data and ABO and Rh blood groups. Results: GDM was diagnosed in 6.6% of women. Age and body mass index were significantly associated with GDM. There were no differences in Rh blood groups (p=0.604), while ABO groups were different between GDM and controls. B antigen was present in 3% of GDM women and in 10.8% of controls (p=0.037), with an odds ratio of 0.25 after adjusting for other associated risk factors (p=0.06). Conclusions: ABO group is suggested as a possible protector marker for GDM.Background: ABO and Rhesus blood systems are associated with type 2 Diabetes Mellitus (DM2). Gestational Diabetes (GDM) is a model to study DM. Aim: To study the association between GDM and ABO and Rhesus groups. Material and methods: A retrospective cohort study was performed in 1078 women who gave birth to a singleton in Talca Regional Hospital, Chile, during 2008. We analyzed personal, obstetric, medical data and ABO and Rh blood groups. Results: GDM was diagnosed in 6.6% of women. Age and body mass index were significantly associated with GDM. There were no differences in Rh blood groups (p=0.604), while ABO groups were different between GDM and controls. B antigen was present in 3% of GDM women and in 10.8% of controls (p=0.037), with an odds ratio of 0.25 after adjusting for other associated risk factors (p=0.06). Conclusions: ABO group is suggested as a possible protector marker for GDM.