Gestational diabetes management: when to escalate from diet to metformin or insulin?

A 32-year-old woman diagnosed with GDM at 26 weeks has fasting glucose 105-115 mg/dL despite 2 weeks of dietary modification and moderate exercise. What are the target glucose levels (fasting, 1-hr, 2-hr post-meal) in GDM? When do we initiate pharmacotherapy, and how do we choose between metformin (considering long-term offspring outcomes) and insulin? What are the monitoring and delivery planning considerations?