A 32-year-old multiparous woman at 26 weeks gestation is found to have gestational diabetes on OGTT (2-hour value 168 mg/dL). Her antenatal records show a prior macrosomic infant (birth weight 4.2 kg) and a history of polycystic ovary syndrome. Which maternal metabolic finding best distinguishes women with GDM who will have adverse fetal outcomes from those with uncomplicated pregnancies?