For the past few years, George King, the director of research at Boston’s Joslin Diabetes Center, has been following a medical mystery that has flown under the public-health radar—even, he told me, among most other diabetes experts. He and his colleagues have been alarmed by the skyrocketing rates of gestational diabetes they’ve seen among Chinese American populations, which mirror a similar phenomenon in mainland China and Taiwan. For years, gestational diabetes has been ticking upward in