The 24-year-old was filmed by her former partner, Sean Hanlon, 26, carrying out the appalling act just a month before giving birth to her son, Jonathon.
Incredibly, when Mr Hanlon showed footage to the police, officers refused to press charges and said no offence had been committed.
Sean filmed the video clip in the couple's living room in October last year, just weeks before Thorpe’s due date.
The footage shows Thorpe, from Chelmsley Wood, Birmingham, lifting her top up to expose her baby bump.
She then walks over to a toolbox, grabs a steel claw hammer and hits the side of her stomach twice, seemingly oblivious to the damage she could be causing to her unborn baby.
Sean said he was appalled at the police’s decision not to press charges, adding: “If there was a film of someone hitting a baby with a hammer they would be in handcuffs in ten seconds.
“I was completely shocked, but Heather didn’t seem to care. I asked her what the hell she was doing but she just told me to stop over-reacting.”
The couple later split and Sean showed police the video clip to demonstrate her character after she accused him of domestic violence.
He added: “I couldn’t believe it when police didn’t press charges. I also sent the video to social services who have been sending someone round to see her regularly, but anyone can put an act on in front of them.”
Thorpe was believed to be eight months’ pregnant when the video was made. In a report by West Midlands Police, a social worker estimates that on a scale of 1-10 (1 being the softest, 10 being the hardest) the blows are around 5 or 6.
This isn’t the first time Heather Thorpe has hit the headlines. Only recently she appeared in newspapers boasting about how Jonathon, now one, could walk and talk by seven months old.
Sean released the video in response to the articles.
Thorpe – who is currently living with her parents - refused to comment.