BEIJING — Births in China dropped to the lowest level in almost 60 years in 2018, signaling the country’s looser two-child policy has done little to reverse its slowing birthrate, and worsening the outlook for growth in the world’s second-largest economy. The number of babies born last year fell by some 2 million from 2017, to 15.23 million, data from the National Bureau of Statistics showed on Monday.