Pregnant women with gestational diabetes mellitus undergoing glucocorticoid treatment to prevent neonatal respiratory distress syndrome could have increased glucose level. We performed a retrospective study and reviewed gestational diabetic women who received an intramuscular dexamethasone injection (6 mg, every 12 hours, 4 times) in our hospital between December 2018 and June 2020. Eligible pregnant women were assigned to the study group (with simultaneous subcutaneous insulin detemir injection, 2-4 units per day) or the control group (without insulin detemir injection). The fasting and 2-hour postprandial blood glucose levels were measured before and on days 1, 2, and 3 after the insulin detemir injection. The changes in their blood glucose levels were compared before and after the drug administrations as well as between the 2 groups. A total of 104 pregnant women were analyzed, including 48 women in the study group and 56 women in the control group. The blood glucose levels increased, with the peak levels occurring on the next day, after the dexamethasone administration in both groups. Compared with the control group, the study group had lower 2-hour postprandial blood glucose levels on days 2 and 3 after the insulin detemir injection (P < .05). There were no statistically significant differences in the fasting blood glucose levels between the 2 groups. Dexamethasone administration increased the blood glucose levels in the pregnant women with gestational diabetes mellitus. A short-course of low-dose insulin detemir administration effectively lowered the blood glucose levels in these women.