Nokuthula Gumede, 23, and her boyfriend Fernando Sidanvo, 35, have been held in police custody since their arrests last week.
The baby was found dead and mutilated in a shack in Diepsloot, north of Johannesburg, on November 10.
Captain Tsekiso Mofokeng said at the time that rats bit off the baby’s foot while it was alone in a shack.
Gumede was arrested in a Randburg tavern last Tuesday. Sidanvo was arrested in Diepsloot the next day.
The baby’s two-year-old sister was placed under the care of social workers last Thursday because her grandmother was being investigated, said Mofokeng.
“There is a possibility of the grandmother being charged with child negligence, but we are still investigating.”
Mofokeng said the toddlers were initially in her care, and she was not supposed to give them to their mother, who allegedly often neglected them.