China's population shrank last year for the first time in 70 years, experts said, warning of a "demographic crisis" that puts pressure on the country's slowing economy.
The world's most populous nation of some 1.4 billion for decades limited most families to one child in an attempt to keep population growth sustainable.
But since 2016 it has allowed couples to have two children in response to concerns about an ageing society and shrinking workforce.
The number of live births nationwide in 2018 fell by 2.5 million year-on-year, contrary to a predicted increase of 790,000 births, according to analysis by US-based academic Yi Fuxian.
Yi, a researcher at the University of Wisconsin-Madison, is a long-term critic of the one-child policy and his work has previously gained traction among the country's leadership.
Enforced through fines but notorious for cases of forced abortions and sterilisation, the one-child policy caused birth rates to plummet after it was introduced in 1979.
However childbirths have not increased as much as forecast since the two-child policy came into force and there has been rising speculation the government will further ease restrictions.
Last year marked a "historic turning point for the Chinese population", said Yi, who studied publicly available data on births in towns and villages across China.
This downward trend may be irreversible, he cautioned, due to factors such as a decrease in the number of women of childbearing age and the reluctance of couples to have children due to rising education, health and housing costs.