Marian Scott, 56, said she saw a large crowd of people gathered around the victim, who was lying on her back in the carpark in front of the Co-op, near to the Hazrath Shahjalal Jamie Masjid mosque.
Scott said a man was bleeding heavily from his head, which he held in his hands.
Police are appealing for information and have released a CCTV image of a man they would like to speak to in connection with the assault, which happened at around 9.40pm on August 6.
Investigating officer Pc Richard Armitage said: "This racially aggravated assault had absolutely devastating consequences for the victim, who lost her baby as a result of the attack. Our thoughts are with her and her family at this incredibly difficult and sad time.
"A thorough investigation is under way and we would like to speak to the man pictured as he may have information which could help with the investigation."