BEIJING • China undercounted the number of children born in 2000 to 2010 by at least 11.6 million - equivalent to Belgium's population - partly because of its stringent one-child policy.

The latest statistical yearbook released by the government puts the number of children born during that period at 172.5 million, well above the 160.9 million in that age group recorded in the 2010 census.

The difference could be the result of some parents failing to register births to avoid punishment for breaching the one-child policy.

China started allowing all couples to have a second child only in 2016, meaning some parents would not officially report a newborn if they were over the quota until the child turned six and needed to register for school, said independent demographer He Yafu.

About 57 per cent of the children later registered were girls, indicating that the discrepancy could also be partly linked to parents not reporting girls so they could continue to try for a boy.

In addition, the 2010 census was conducted on Nov 1 that year, and so would have missed births in the last two months of the year. Census surveys also typically do not include people who have died or emigrated in the intervening years.

The revisions show how hard it is to accurately count the number of people in the world's most populous country.

Birth rates for the years 2011 to 2017 were also revised upwards in the latest statistical yearbook, suggesting that the problem of undercounting the number of children likely continued after 2010.

However, with China now effectively abandoning limits on family size, there could be less of a discrepancy in future.

While the cap for most families is now set at three children, there are no penalties for exceeding it. Still, the number of births in China is expected to continue declining, and the total population could start shrinking as early as this year.

The results of a once-in-a-decade census announced in May showed that the population grew at its slowest rate since the 1960s.

The scrapping of the one-child policy - one of the world's strictest family planning rules - failed to bring about a hoped-for baby boom, as the cost of living rises and women increasingly make their own family planning choices.

China last year recorded 8.52 births per 1,000 people, the statistical yearbook shows - the lowest figure since the yearbook data began in 1978. It was a marked fall from 2019's 10.41, and the lowest figure since the Communist Party of China was founded in 1949.

The latest data highlights the looming demographic crisis caused by a rapidly ageing workforce, a slowing economy and weak population growth.

The yearbook also showed the number of marriages registered last year hit a 17-year low, with only 8.14 million couples tying the knot. The number of divorces fell for the first time in at least 30 years, after a mandatory 30-day "cooling-off period" for divorcing couples was implemented.

The data highlights social trends troubling the Beijing leadership.

BLOOMBERG, AGENCE FRANCE-PRESSE