A 40-year-old man is being questioned over the murder of mother-to-be Krystal Hart, who was gunned down in her own home.
Scotland Yard said the man was arrested after walking into a police station in south London.
Ms Hart, 22, was shot twice in the head in her flat in Clapham, south-west London on Friday morning.
Police had been looking for a white man aged between 30 and 50 over the killing, which detectives have said may be linked to a dispute with a neighbour.
Ms Hart's devastated mother Deborah Penfold said her daughter was "indescribable in every way", adding that her death left "a gap which will never be filled".
She said the murder would not extinguish the "light" of her daughter's memory.
Officers and paramedics were called to the flat shortly after 11am on Friday, but Ms Hart, who was three-months pregnant, was pronounced dead at the scene.
A post-mortem examination showed that she died from two gunshot wounds to the head.
Detectives took the unusual step of revealing that they believed the killing may be linked to a row dating back "many months" although police no longer believe it was sparked by a dispute over parking.
Ms Hart's stepfather Clive Lawrence, 50, has told how he installed cameras in Ms Hart's flat because of the dispute.