Mexican state officials have used DNA tests to confirm the identity of the baby's father after she originally told hospital workers it was her 17-year-old boyfriend.
Although results won't be known for a fortnight, it has emerged that the stepfather's name is on the baby's birth certificate.
Also, state prosecutors did a study of the girl, which helped them to conclude that she is between 12 and 13 years old.
MailOnline is reporting that it was 'highly likely' that the girl, called Dafne, was not the age her mother claimed she was when she went into hospital with her daughter in labour.
The reason for the lie could be that her mother may have been trying to get her into school for the first time because she had not gone until now. Another more sinister reason, according to the news site, could have been if, as has been claimed by neighbours, the mother is a prostitute who is trying to turn Dafne into a sex worker too, a nine-year-old could command a higher fee from clients than a teenager.
Lina Ginzalez Corona, head spokesman for the Prosecutor's office in the state of Jaricso, said there had so far been no arrests and no charges filed.
He said: "The girl is still insisting that it is her boyfriend who got her pregnant. The problem is that we don't know where the guy is so we are looking for him."
If the father turns out to be the girl's stepfather as some have claimed, he would be tried as an adult and could be put away for between 12 and 20 years.
Dafne's baby was born by C-section on January 27 in Zoquipan Hospital, in Zapopan, Mexico's western Jalisco state weighting 5.95lbs. Both girls are doing fine and in a healthy condition. According to Dafne's mother the girl was eight at the time she got pregnant but she didn't realise she was expecting until she was seven months gone.
It is also reported that Dafne has been surgically fitted with an implant that will stop her getting pregnant.
Her mother has reported the Zoquipan Hospital to the local human rights commission because she claimed she did not approve the procedure.
But in a press conference health commissioner Munoz Serrano said that he had a note in which she gave her written permission.