The Chinese government says it is drafting new laws to tackle the growing gender imbalance caused by the widespread abortion of female foetuses.
The practice is already banned, but new rules are expected to set out specific punishments for parents and doctors.
China's Family Planning Association (CFPC) has revealed the extent of the imbalance - in one city there are eight young boys for every five girls.
Experts fear the phenomenon could have unpredictable social consequences.
Some believe that with millions of men unable to find a wife, there could be risks of increasing anti-social and violent behaviour.
Boys preferred
China's one-child policy, and a traditional preference for male heirs, has led many couples to try to ensure that their single offspring is a boy.
Some pay for illegal ultrasound tests to discover the sex of a foetus, and abort it if it is female.
"The root cause is traditional thinking that boys are better than girls, especially in poverty-stricken areas," Song Jiang, a population expert at Beijing's Renmin University, told the Xinhua news agency.
"Those people expect boys to support the family."
On Friday it was revealed that the eastern city of Lianyungang had the most skewed population. Among children under four years old, there are 163.5 boys for every 100 girls.
Ninety-nine cities had gender ratios higher than 125, state-run news agency Xinhua quoted the CFPA as saying in a report.
The UN recommends a gender ratio of no more than 107.
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