A 26-year-old woman who was approximately eight months’ pregnant suffered fatal stab wounds in the attack and went into cardiac arrest, Metropolitan Police said in a statement.
Despite the efforts of paramedics at the scene, the woman — named as Kelly Mary Fauvrelle — died inside a property at Thornton Heath, south London, at about 3:30 a.m.
“This is a horrific incident in which a young mother has lost her life and her child is critically ill,” Detective Chief Inspector Mick Norman said in a statement.
“Our sympathies go out to her devastated family; they are being supported by specially trained officers following this awful event.”
The child, whose gender has yet to be revealed, remains in a critical condition in hospital.
A 29-year-old man has been arrested on suspicion of murder and is currently in custody, the Metropolitan Police said.
Police arrested a 37-year-old man earlier but he was released under investigation, pending further inquiries.
“At this stage officers retain an open mind as to any motive,” the police said.
According to Britain’s Press Association (PA) news agency, an air ambulance, two ambulance crews and two response cars were sent to the scene as medics fought to save the mother.
One neighbor described the victim as “just a nice girl,” according to PA, adding that despite being awake at the time of the incident she did not hear or see any signs of an attack.
“I came back home between 1 a.m. and 2 a.m., I was even talking on the phone in my backyard,” she said, according to PA.
London Mayor Sadiq Khan tweeted that “violence against woman has no place in our city, and horrific murders in the home like this show the scale of the problem we face.”
He added: “My heart goes out to this innocent child, and to the mother they have so tragically lost. @MetPoliceUK are investigating — please help if you can.”
Police said a post-portem examination would be held “in due course.”
Knife crime has become a national crisis in the UK, where police recorded a total of 40,577 offenses involving a knife or sharp instrument last year — 10,000 more than in 2011.
Few weekends pass without reports of violence in the country, while flowers laid on London street corners serve as a near-daily reminder of its results.
Earlier in June two teenagers died moments apart on a bloody Friday night in London when four people were stabbed and a teenager was shot dead in a spate of separate incidents.