Police found the body of the child at an address in Clapton, on Friday, after reports of a knife incident shortly after 3pm.
A woman, believed to be the girl's mother, was arrested on suspicion of murder. She has since been sectioned under the Mental Health Act and is in a secure unit.
A post-mortem examination took place yesterday at Poplar mortuary in east London but no formal cause of death was given and detectives are awaiting the results of further tests.
The Metropolitan Police child abuse investigation command is leading the murder inquiry.