The 26-year-old Royal Mail worker was eight months’ pregnant when she was killed in the bedroom of her home in Raymead Avenue, Thornton Heath, London, in the early hours of June 29.
Her baby son – who was named Riley by family members after he was delivered by paramedics – died in hospital on July 3.
A Metropolitan Police statement 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. Inquiries continue.”
The attack took place in Ms Fauvrelle’s bedroom while her whole family was at home and relatives heard her screams at around 3.30am.
Two men have been arrested on suspicion of murder previously.
One, aged 37, was released with no further action, and a second, aged 29, has been 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.