Malorie Bantala, a former University of Bedfordshire student was 32 weeks pregnant when she was attacked.
One man has already been charged but the substantial sum is available for information leading to the arrest and prosecution of a second suspect involved.
Malorie was 32 weeks pregnant when she was attacked and her unborn child could not be saved.
Detective Inspector David Suddick, from the Homicide and Major Crime Command, said: “Although one man has been charged, we have yet to identify the second person involved in this horrific attack.
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“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. If you have any information, please contact us.”
The 21-year-old mum to be was walking down Talfourd Place, Peckham, at around 8pm on Monday, June 15, when she was approached by two men wearing motorbike crash helmets.
They pushed her to the ground and continually kicked and stamped on her stomach.
The suspects then ran off, still wearing the crash helmets, in the direction of Denman Road.
A member of the public called police who administered first aid at the scene. Malorie was taken to a south London hospital where she lost her baby.
The outstanding suspect is described as being in his 20s.
DI Suddick added: “We are also appealing for any information about the victim’s white iPhone 5s which went missing during the attack. It has yet to be traced.”
>Anyone who can help is asked to contact the incident room on 020 8345 3985 or Crimestoppers anonymously on 0800 555 111.