China's migrant population - people who left their hometown to seek employment or education elsewhere - decreased for the first time in about 30 years in 2015, a change that surprised demographers and economists.

The country's migrant population dropped to 247 million at the end of last year, a decrease of 5.68 million, according to the National Bureau of Statistics.

The overall reduction of the migrant population arrived a year earlier than expected, said Li Xunlei, chief economist with Haitong Securities. The number of people moving to cities was dwindling at a pace of about 2 million per year since the peak of 12 million more migrants annually in 2010, Li said.

China's overall working-age population, defined as people aged 16 to 60, fell by a record 4.87 million to 911 million in 2015, the fourth consecutive year of decline.

Because labor, capital and technology are key to economic growth, Li said, a drop in the migrant population coupled with a shrinking labor force will slow growth.

A decreasing labor pool has resulted in increased labor costs, prompting an outflow of investment.

Consumption, another driving force for economic growth, was also slowed by the decline in the working-age population, Li said.

China's shift to a two-child policy has not helped so far. The NBS figures show that the country's newborn population dropped 320,000 from 2014 to 16.55 million last year, instead of an expected rise, according to Huang Wenzheng, a demographer.

The situation is so grim that another demographer, Liang Jianzhang, recently suggested China should encourage women to allow routine artificial insemination, according to media reports.