The case of a pregnant 12-year-old Vietnamese girl shocked China in the past week, highlighting the impact of a gender imbalance that is not expected to improve for some time.

The girl, discovered when she arrived at a Chinese hospital for a pre-natal appointment, was a victim of human trafficking, police said. Public prosecutors in the city of Xuzhou said she had been abducted in her home country before being transported to China and sold in May to a 35-year-old man for 30,000 Chinese yuan ($4,465).

The case, which dominated social media conversation, is a stark reminder of the fallout from a decades-long one-child policy, which was introduced in 1979 to curb a burgeoning population.

A cultural preference for sons meant that female fetuses were aborted and baby girls abandoned, leading China's National Health and Family Planning Commission to acknowledge in 2015 that the country had the world's most serious and widespread gender imbalance.

Late last year, China announced that it would ease family planning restrictions to allow all couples to have two children.

But the gender imbalance will remain over the long term, despite recent improvements in the sex ratio, a family planning commission official warned on Tuesday, according to the state-run China News Service.

In 2015, the birth gender ratio showed a decline in the number of males to females for the seventh straight year, but it was still at 113.5 boys for every 100 girls, Wang Peian, deputy head of the commission, said. Wang did not say when the country's gender ratio would eventually even out.