Chinese warehouse worker Liu Fei was fined 330,000 yuan (S$68,000), or 14 times her yearly wage, for having a second child.
Her failure to pay the fine means the boy has no access to basic rights like schooling and health care.
Ms Liu's desperation prompted a fruitless attempt to sell her kidney.
Their dilemma has now triggered a rare legal battle against the police for denying the boy a "hukou" - household registration - due to strict family planning laws. The case will be heard in a Beijing court today, Reuters reported.
Ms Liu's inability to pay the fine has left her son, Xiaojie, without an official identity. Family planning officials in Beijing told Liu in 2011 she would not be able to obtain a"hukou" if she did not pay up.
"When I saw (the fine), I thought this was inconceivable," a tearful Ms Liu, 41, told Reuters.
"I had no idea I would be fined so much. If I had known, I would never have given birth."
Her ordeal underscores the punitive nature of China's family planning policy, beyond the more well-known stories of forced abortions and sterilisations, and highlights the plight of an estimated 13 million undocumented children, known as"black children".
In China, "black" implies something illegal, outside of the mainstream or unofficial.
She asked for her and her son to be identified by pseudonyms to avoid drawing attention.
Beijing gives local governments a wide mandate to enforce the one-child policy.
Even as China relaxes the rules, allowing millions of families to have a second child, government encroachment into family matters will continue. On Tuesday, a group of 10 lawyers and academics sent letters to four government agencies, including the State Council, China's Cabinet, and the Ministry of Public Security, urging them to review requirements to file for a "hukou" and scrap the need for fines to be paid first.
"Without this document, you're just like a puppy or a kitten, raised at home," said Ms Liu's lawyer Huang Yizhi.
"This is related to a person's most fundamental right, but because she didn't pay the social support fee, the police refused to give him (Xiaojie) a hukou. I think this condition is certainly unreasonable and illegal."
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