A 32-year-old multigravida with gestational diabetes mellitus diagnosed at 26 weeks is on dietary management and self-monitoring of blood glucose for 2 weeks. Her fasting glucose readings are consistently 105–110 mg/dL and 2-hour postprandial values are 155–165 mg/dL, despite strict adherence to a 1800 kcal/day diet with appropriate macronutrient distribution. What is the most appropriate next step in management?