Pregnant Kelly Mary Fauvrelle, aged 26, was stabbed to death in Croydon, south London, on Saturday.
Her baby boy, named Riley by relatives, was delivered at the scene by paramedics and taken to hospital in a critical condition, but Scotland yard announced today that he died in the early hours of today.
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Ms Fauvrelle was eight months pregnant when she was attacked.
CCTV footage of a man seen running away from the house where Ms Fauvrelle was found has been released by the police today.
It shows a figure walking towards Ms Fauvrelle's home at around 3.15am on Saturday, before running away 10 minutes later.
Two men have been arrested on suspicion of murder.
A 37-year-old man has been released under investigation while a 29-year-old man has bailed until next month.