Gestational Diabetes Mellitus (GDM) is a metabolic complication of pregnancy which involves β-cell dysfunction and Insulin Resistance (IR). GDM is a pathology associated with a significant risk of macrosomia and metabolic complications for the offspring. The mechanisms underlying GDM pathogenesis remains unclear. The aim of our study was to analyse the factors related to insulin resistance and β-cell dysfunction involved in the pathophysiology of GDM complicated with macrosomia compared with GDM without macrosomia. The patients were recruited as part of a case-control study: Patients with GDM without macrosomia, patients with GDM complicated with macrosomia and healthy gestational controls. Blood samples were collected at the third trimester of pregnancy and tested for adiponectin, leptin, insulin, proinsulin and C-peptide, Homeostatic Model Assessment-IR (HOMA-IR), steady state β-cell function (HOMA%B), insulin sensitivity (HOMA%S) and Body Mass Index (BMI). All patients diagnosed with GDM showed: An impairment in HOMA%B and a decrease in C-peptide maternal serum concentration. HOMA%B presented a stronger positive correlation with pre-pregnancy BMI and maternal weight gain, and a stronger negative correlation with adiponectin. Our study suggests that the different correlations among the factors involved in the pathogenesis of GDM may explain the evolution of GDM pregnancy to macrosomia.