Authorities in Beijing said last week they had seized foetus gender testing kits sent from the United States among parcels mailed to Chinese consumers who placed orders online.
This was the first time such items have been found during entry-exit inspection in Beijing.
The Beijing Entry-Exit Inspection and Quarantine Bureau said they intercepted three packs of such testing kits.
The bureau said it will destroy the testing kits and demand the online shopping platform refrain from mailing such items to China.
The testing kits can allegedly predict the sex of a foetus in the first five to 10 weeks of pregnancy through a few drops of urine.
Last year, four boxes of similar products were seized in south-west China's Chongqing municipality, China News Service reported.
Prenatal gender testing for non-medical reasons is banned in China, out of fears that predicting the sex of the foetuses might lead to an increased number of abortions and negatively impact the gender imbalance among newborns.
Individuals and organisations that perform medically unnecessary prenatal gender predictions or sex-selective abortions will be fined up to 30,000 yuan (S$6,100), according to a revised regulation that took effect in May.
The regulation aims to tackle China's gender imbalance, a result of pre-birth gender prediction and sex-selective abortions driven by a cultural preference for sons.
Last year, China's sex ratio, which represents the number of men for every 100 women, stood at 113.5, a ninth consecutive yearly decrease.
A normal range is between 103 and 107.
Goals by the latest government health guidelines include a balanced sex ratio at birth by 2030.
Qiao Xiaochun, a population scientist at Peking University, said the latest universal two-child policy, which took effect at the beginning of the year, would gradually help redress the skewed gender ratio.
"As family planning policies have been eased in the country, a preference for boys is expected to change as well," Professor Qiao said, adding that it would take time for the sex ratio to reach a normal range.