Neighbours said that three women, one of whom was pregnant, lived at the house on Raymead Avenue and had a small dog.
Reacting to the murder on Twitter, Sadiq Khan, London mayor, said: “Violence against women is endemic in society and devastating murders in the home, like this one, show the scale of the problem we face.
“My prayers are with this innocent child, and with the mother it has so tragically lost.”
A London Ambulance Service spokesman said: “We sent an incident response officer, two ambulance crews, two medics in response cars and an advanced paramedic to the scene. We also dispatched London’s Air Ambulance.
“We took one person to hospital as a priority. Sadly, despite the extensive efforts of medics, another person died at the scene.”