According to reports, the teenage girl committed the murder on the orders of a 35-year-old woman who wanted to keep the baby for herself, Daily Mail reports.
Fabiana Santana, 23, was eight months pregnant when the girl attacked her.
Santana’s seven-year-old son Gustavo was then drowned when he tried to save her in Porto Velho, Brazil.
She had been bludgeoned around the head with an iron bar, leaving her face disfigured and stabbed to death.
Police say the 13-year-old girl carried out the attack with a 15-year-old boy on the orders of Catia Rabelo who masterminded the plan.
She was arrested on Wednesday after going on the run and has allegedly confessed to inciting the teenagers to commit the brutal killings.
It is believed Rabelo intended to trick her boyfriend, a gold prospector, into believing she had given birth to his child.
Remarkably, police found the baby alive in the care of the 15-year-old accomplice at his family’s home.
The 15-year-old’s mother, who was not present at the murders, had fled from the home.
The child, weighing 3.9lbs, is under observation at Porto Base Old Hospital and is in good health despite his horrifically violent birth.
