Hi, Welcome to HCM, Your wife is almost full term pregnant with Diabetes type I and taking Insulin, I presume she must be managed with utmost care and her blood sugar level maintained well. Now the question of Normal Vaginal delivery or planned C section the decision depends on various factors taken in considerations as chances large for gestational age (macrosomic) are more in such cases and also depends on your pelvic size and hence if vaginal delivery has risks of obstructed labor and shoulder dystocia. Your Gynecologist takes all considerations before reaching to conclusion what is best for you. Certainly there are ways to monitor size of baby and calculate expected weight also. In general C Section is safe in such cases. Be Positive do not worry wish you safe mother hood. Take care Good Luck.