About a third of China’s regions posted economic growth of less than 7 percent in the year’s first quarter, data showed, trailing the wealthiest regions, and adding to fears the nation is headed for its worst downturn in a quarter of a century. In a worrying sign of a yawning wealth gap, the difference between China’s fastest- and slowest-growing regions spiked to a whopping 8.8 percentage points in the first quarter, media and local government figures showed.