Police were called to an address in south London at 3:30 a.m., where they found a wounded 26-year-old pregnant woman. Her baby was delivered, taken to hospital and was in a critical condition.
Late on Saturday night, police were informed of an incident in the east of the British capital and found a man suffering from stab injuries. The man, believed to be in his late-20s, was pronounced dead at the scene.
The incidents appeared to be unrelated.
A 37-year-old man was arrested on suspicion of the murder of the woman, while police were seeking more information about the second stabbing.