A man looks across Deep Bay towards the Chinese mainland city of Shenzhen's (back) skyline from Hong Kong on September 12, 2018, in this file photo.
Hong Kong CNN  — 

A 12-year-old girl has been caught in the Chinese city of Shenzhen trying to smuggle dozens of blood samples from pregnant women into neighboring Hong Kong, apparently for sex testing, state media reported.

The girl was detained at Shenzhen’s Luohu port, according to the People’s Daily. The incident occurred on February 23, but it only became widely known after it was reported in local media Thursday.

Sex verification is illegal for pregnant women in mainland China unless it’s required for medical purposes. The traditional desire for a male heir prompted abortions and even infanticide under the nation’s previous one-child policy.

The girl said someone had offered to pay her to bring the samples across the border.

“Cross-border students basically don’t bring anything other than books, stationery and snacks, so their schoolbags usually look lean. But we saw that hers looked so full that it might burst, so we scanned her bag,” said a Luohu port staff member quoted by the People’s Daily.

Shenzhen’s immigration and customs department declined to comment on the case.

The People’s Daily said it was the second case of its kind this year. Authorities say the use of underage people to transport cross-border blood samples is become increasingly common.

The cases could raise concerns that traditional families are going to extreme lengths to ensure they have boys, as they did during the one-child policy.

The one-child policy was introduced in 1979 due to concerns by the Communist Party leadership that the population would soon spiral out of control.

Though it was axed in 2015, the country’s birth rate has not risen as officials hoped. China currently faces a massive gender imbalance, a shrinking pool of young people left to support their aging parents and grandparents, and a contracting workforce.

China has tried to combat its demographic challenge by loosening centrally controlled family planning policies and encouraging citizens to have more children. So far, it does not appear to have worked.

Population growth slowed in 2018 with 15.23 million live births, a decline of 2 million from the year before, according to the National Bureau of Statistics.

Many middle-class families say cost is one of the main reasons they do not plan on having more children.

CNN’s Ben Westcott, Nanlin Fang and journalist Chermaine Lee contributed to this report