The investigation into the missing child was sparked by a case review into the death of another child at an address in Barnet in 2015.
A 50-year-old woman was arrested in Fryent Crescent, West Hendon in north London, on Monday. Later, a woman, 31, was detained in Luton, Bedfordshire.
Both were held on suspicion of murder.
Following a search of the property in West Hendon on Tuesday, police revealed human remains had been found.
The two women were then re-arrested along with a 52-year-old man on suspicion of preventing a lawful and decent burial.
The older woman and man have been bailed until mid-July while the younger woman remains in police custody.
A post-mortem examination on the remains at Great Ormond Street Hospital was inconclusive with regard to the cause of death.
A Metropolitan Police spokesman said: "Further tests will be undertaken and this could take a number of weeks."
He said the death of the child last year was thought at the time not to be suspicious.
He said the case had been referred to its Directorate of Professional Standards (DPS) and also to the Independent Police Complaints Commission (IPCC).
Two women have been arrested after an investigation into the disappearance of a baby in 2004 led to the discovery of human remains.