Gestational diabetes mellitus (GDM) is a metabolic complication of pregnancy. The pathogenesis of GDM is considered to involve β-cell dysfunction and insulin resistance (IR). GDM is associated with a significant risk of macrosomia in addition to a high probability of metabolic complications for the offspring. The precise mechanism underlying GDM remains unclear. The aim of the present study was to analyse the factors associated with insulin resistance and β-cell dysfunction involved in the pathophysiology of GDM complicated with macrosomia compared with GDM without macrosomia. In addition, another aim of the present study was to assess the relationship between GDM complicated with macrosomia and anthropometric, clinical and paraclinical parameters. The following group of 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) were also calculated. All patients diagnosed with GDM showed an impairment in HOMA%B and a decrease in C-peptide maternal serum concentration. Additionally, diabetic status leading to the birth of offspring with macrosomia did not induce changes in the maternal serum levels of insulin, proinsulin, adiponectin or leptin, which was also the case in patients with GDM but not macrosomia. HOMA%B presented a stronger positive correlation with pre-pregnancy BMI and maternal weight gain, and a stronger negative correlation with adiponectin. Furthermore, HOMA%S in this group exhibited strong positive correlations with maternal serum levels of high-density lipoprotein cholesterol (HDL) and aspartate aminotransferase, and a strong negative correlation with pre-pregnancy BMI. In the same patients, HOMA-IR was also found to have a high negative correlation with HDL levels, and highly positive correlations with gestational age and triglyceride levels. In conclusion, the present study suggests that the different correlations among the factors involved in the pathogenesis of GDM may explain the evolution of GDM pregnancy to macrosomia.