Kelly Mary Fauvrelle, 26, was stabbed in the bedroom of her home in Thornton Heath, Croydon, south London, last month.
She died at the scene, but emergency services were able to deliver her baby boy.
The child, who family members named Riley, was in a critical condition before he died in hospital on Wednesday 3 July.
The Royal Mail worker, who was eight months pregnant, was found by police and paramedics with stab wounds at 3.30am on Saturday 29 June at the property in Raymead Avenue.
Her family was also at home at the time and relatives heard her screams.
A statement from Scotland Yard said: “On Thursday July 11, detectives investigating the murder of Kelly Mary Fauvrelle and her baby son, Riley, arrested a 25-year-old male on suspicion of murder.
“The male has been taken to a central London police station where he remains at this time. Enquiries continue.”
Two other men have previously been arrested on suspicion of murder.
One, aged 37, was freed with no further action, and a second, aged 29, was released on bail until early August.
:: Anyone with information is asked to call the police incident room on 020 8721 4005 or Crimestoppers anonymously on 0800 555111.