Gestational diabetes mellitus (GDM) is a glucose intolerance resulting in hyperglycemia with onset or first recognition during pregnancy. High fat diet (HFD) is one of the contributing factors to GDM. This study evaluated the contribution of HFD in differential adipose tissue (AT) expansion and subsequent development of GDM in Wistar rats. Pregnant and non-pregnant rats were given streptozotocin (STZ) by a single intraperitoneal injection or HFD throughout the experiment. The animals were sacrificed on day 1, 8, 15, or 21 of the experiment. Blood, adipose tissue and pancreas were collected and analyzed. In this study, STZ treated animals had a significant (p<0.05) increase in serum glucose and a decrease in insulin, without changes in the size of adipocytes. The levels of both serum glucose and insulin were significantly high in HFD fed animals (p<0.05); being higher in pregnant (p<0.05) than non-pregnant rats. The increase in glucose and insulin levels was associated with increase in the size (hypertrophy) than number (hyperplasia) of adipocytes. The increase in size of adipocytes was higher in viscera (VAT) than subcutaneous (SAT) adipose tissue and related to insulin resistance and GDM development. Histologically, the number of β-cells was decreased and deformed in STZ groups while maintained in HFD groups in both pregnant and non-pregnant animals. The findings of this study show that, intake of HFD during pregnancy may lead to AT expansion and thus insulin resistance, which is one of the risk factors for the hyperglycemia and development of gestational diabetes mellitus.