Riley, the child of Marie Kelly Fauvrelle, a woman of Mauritian origin, has died.
It comes after police investigating the murder of the 26 year old in Thornton Heath released CCTV footage of a man they wish to trace.
Ms Fauvrelle, 26, who was eight months pregnant, was found by police and paramedics with stab wounds at 3.30am on Saturday 29 June at an address at Raymead Avenue, Thornton Heath.
She died at the scene, but ambulance staff were able to deliver her baby boy.
The child was in a critical condition before his death on Wednesday.
New CCTV shows a man walking towards Ms Fauvrelle's address at around 3.15am on Saturday.
Just over 10 minutes later, the same male can be seen running back along Raymead Avenue, away from Kelly's address, toward Colliers Water Lane.
Detective Chief Inspector Mick Norman, who is leading the investigation, said:
The release of this footage is the next step in what has been a fast-paced, and extremely challenging investigation. We need to identify the man shown in the footage urgently, even if only to eliminate him from our enquiries. I need to hear from anyone who knows who he is. If this is you, it is imperative that you contact my team immediately.
He added that the force was unable to say whether Ms Fauvrelle's attacker was known to her and officers are keeping an open mind. Mr Norman said:
A dedicated team of detectives are working day and night on this investigation. We have assured Kelly's family - and I want to assure local residents and the wider public too - that we are doing absolutely everything in our power to find the person responsible. I am aware that there is some speculation about whether Kelly's attacker was known to her. At this stage we simply are not in a position to say and we must retain an open mind. One of the key aims of my investigation is to build a complete picture of Kelly's life and the people with whom she was in contact, but I also need to consider other possible scenarios. Members of the public may have useful information, and I am today reiterating my appeal for anyone who may know anything that will assist the investigation to make contact as soon as possible.
Two men have been arrested on suspicion of murder. One, aged 37, was released under investigation on Sunday, and a second, aged 29, has been released on bail until early August.
Watch the CCTV footage below: