The world's second-largest economy reported an increase of 72 million people in the last 10 years in the once-a-decade census, to a total of 1.1411 billion.
But the National Bureau of Statistics said annual growth over the last decade averaged 0.53 percent, down 0.04 percent in the previous decade. The slowdown bolsters evidence of what economists refer to as a demographic time bomb, where many Chinese people could grow old before they grow rich.
Any slowdown is politically sensitive for the ruling Communist Party, which garners much of its legitimacy from a booming economy and social prosperity. For many, having the largest population and standing army in the world are a source of everyday national pride.
The report said President Xi Jinping hailed the census, in which more than 1 billion Chinese citizens participated, as "a major survey of national conditions and strength in the new era and a major event for the Party and the country."
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The highly-anticipated census, which saw officials conduct door-to-door data collection, was initially due to be published in early April. Instead it was delayed as it required "more preparation work," China's National Bureau of Statistics told The Financial Times newspaper.
NBC News did not receive a reply to a request for comment from the National Bureau of Statistics on the report's delay.
Chinese state media and official bodies pushed back after the newspaper reported the census was set to show a population decline for the first time since a famine that killed millions four decades ago.
Speaking ahead of the census release, Yi Fu-Xian, a senior reproductive scientist at the University of Wisconsin-Madison, said a population decline hitting sooner than expected would impact China's "economic, social, science and technology, national defense, foreign affairs and other policies."
It would also have geopolitical implications if neighboring India quickly surpassed China as the world's most populous country, he added, causing Beijing to lose face and be overtaken by a rival.
"The Chinese authorities have so far been afraid to publish the main data of the census, probably because the data do not match expectations. The census results will shock the world," Yi, author of "Big Country with an Empty Nest," said.
With China at risk of entering an irreversible population slide, policymakers are under pressure to come up with family-planning incentives and arrest the falling birth rate.
In an encouraging sign for Chinese policymakers, the proportion of people 14 and under increased to 17.95 percent — an increase from 16.6 percent a decade ago, a low figure caused by the country's decades-old one-child policy, which was revoked in 2016.
However, despite an increase in young people, the number of older people also grew to 18.7 percent of the total, up from 13.26 percent a decade ago and about 10 percent in 2000.
Any future reduction in the proportion of young people could add increased burden to China's working-age population and weigh on productivity.
Kent Deng, professor of economic history at the London School of Economics, said China's population had shown a "clear trend" of decline at a rate between 3 and 4 percent per year over the last 50 years.
He said the dip in population would lead to a decrease in China's labor force and working population and could see more extensions to the country's retirement age.
"Less children will impact on the military," he added, and possibly the long-term stability of the ruling Communist Party.
China's birth rate has continued to fall despite public campaigns and incentives.
That is in part because urban couples, despite parental pressure to have children, increasingly value their independence and careers more than raising a family.
"I have no children and do not plan to have children," Siqi Xiang, 23, a media worker in Beijing, said.
"The cost is too high," she added. Along with "the awakening of female consciousness, we no longer think that fertility is a life-task that must be completed like the generation of our parents."
For Tang Li, 41, a legal worker from the southeastern port city of Xiamen and a mother of one, a second baby is not on her agenda "no matter what policy the government have," she said.
Tang said if she were younger and had fewer financial burdens, she may have considered another child but acknowledged that professional development, quality of life and changing expectations among women had a role to play in families opting for fewer offspring.
At the other end of the social spectrum, an aging population was also a worry, Tang added, with grandparents undertaking substantial child care responsibilities.
"There is nothing we can do. I just decide to save more money by ourselves and exercise better," she added. "I am a staunch patriot ... it would be good to rely on the country but it will definitely not work to rely solely on children."
An unusually frank report from the People's Bank of China in March urged the ruling Communist Party to "fully liberalize and encourage childbirth," it said, citing fears over pension deficits and an expensive, aging population to maintain.
The report also said China could lose out to neighboring India and to the United States as it nears the end of its 'demographic divide' — an economic benefit where the working age population outnumbers the nonworking population.
The research noted that the U.S. also benefits from mass immigration, unlike China.
Last month, the U.S. Census Bureau published its own data that showed overall the American population stood at 331,449,281 as of April 2020 — although slowing, a 7.4 percent increase over the previous decade. The American birth rate also plunged last year, government data released in May found.
"China's aging problem is much more serious than officially announced," scientist Yi said, warning increasing retirement ages could put undue pressure on the economy and "lead to social unrest."