Malorie Bantala, 21, was eight months pregnant when she was attacked by two men in Talfourd Place, Peckham, in south-east London, in June.
A 20-year-old man has been charged with child destruction and grievous bodily harm with intent.
But officers have yet to identify the second suspect.
Det Insp David Suddick said: "The victim has recovered physically from her ordeal, but naturally remains devastated at the loss of her baby.
"Somewhere out there is a very violent individual and we hope the offer of this reward may encourage someone to come forward."
He added that the victim's white iPhone 5s went missing during the attack and is yet to be traced.
Ms Bantala's former boyfriend, Kevin Wilson, of Longfield Estate, Bermondsey, south-east London, is due to face trial in November.
A £20,000 reward is being offered to find a "very violent" man involved in an attack which resulted in a heavily pregnant woman losing her baby.