BEIJING—China’s population growth in the decade to 2020 slumped to the least in official records dating back to the 1950s, fuelling pressure on Beijing to ramp up incentives to couples to have more children and avert an irreversible decline. With growth having slowed ever since a one-child policy was introduced in the late 1970s, the 2020 results of the country’s once-a-decade census, published on Tuesday, showed the population of mainland China increased 5.38 percent to 1.41 billion. That compared with an increase of 5.84 percent to 1.34 billion in the 2010 census, and double-digit percentage rises in all of China’s previous six official population surveys dating back to 1953. The number meant China narrowly missed a target it set in 2016 to boost its population to about 1.42 billion by 2020. In 2016, China replaced its one-child policy—initially imposed to halt a population explosion at the time—with a two-child limit. …