This study evaluated the protective effect of Lentinus edodes in rats with streptozotocin-induced gestational diabetes mellitus (STZ-GDM) when administered orally. The rats received from the 1st to the 19th day of gestation daily doses of 100 or 200 mg/kg of lyophilized and reconstituted L. edodes; the animals in the saline control group and diabetic control group received a saline solution (DS). Gestational diabetes mellitus was induced by streptozotocin (80 mg/kg, administered intraperitoneally) on the fourth day of pregnancy; blood glucose > 180 mg/dL was considered to indicate STZ-GDM. L. edodes reduced catalase in plasma. We also observed reduced glucose in plasma, urea, triglycerides, and aspartate aminotransferase. There was a decrease in preimplantation loss when compared with the DS group. The doses of L. edodes used here had a protective effect on the preimplantation parameters in STZGDM. However, the mushroom was not able to reverse the deleterious effects caused by streptozotocin throughout the evolution of pregnancy.