Jordan Roche's victim, who gave birth 16 weeks before the attack, suffered bruising and swelling to her face and had her hair and hair extensions pulled out in the assault.
Newcastle Crown Court heard Roche, 22, also beat up a man who tried to intervene in the violence, which happened outside a house in South Tyneside last August.
Prosecutor Peter Sabiston told the court the victim had been out drinking with a friend in the hours before the attack, which started in a back garden.
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Mr Sabiston said the male victim saw Roche push the woman to the ground and "could hear her screaming" and tried to stop the violence.
But he added: "Mr Roche told him to 'keep the **** out of it, I will knock her clean out'
"He told him to stop and at this point Mr Roche ran at him as he tried to intervene and punched him to the face."
The court heard Roche then resumed his attack on the woman.
Mr Sabiston said: "The defendant kicked her really heard to the head as she lay on the ground, apparently unconscious."
The court heard Roche threw repeated punches at the male while his ex-partner lay on the ground, "unresponsive".
In a victim statement, the woman, who has been left with memory problems, said: "I am in pain.
"Knowing he has assaulted me is shocking. He's a violent man.
"I just want to make a fresh start for the future with me and my baby.
"I fear he may have left me with permanent damage. I just want him to leave me alone."
Roche, of Great North Road, Chester-le-Street, who has a criminal record which includes offences of violence and damage and was on a suspended sentence at the time, admitted two charges of assault.
Judge Tim Gittins has sentenced him to two years behind bars, with a restraining order to keep him away from the victims.
The judge said: "This was, on any view, a serious incident. It was sustained violence and it went on for some time.
"It involved you dragging her about, pulling her hair out and her hair extensions out, causing her injuries to her face and around her eyes.
"You were witnessed kicking her to the face by a man who himself was also subjected to a sustained assault.
"Fortunately, his injuries were less serious but he was left with cuts and bruises.
"The psychological and emotional effect the behaviour had towards the mother of your child and a friend is significant."
Vic Laffey, defending, said Roche's guilty pleas spared the victims from giving evidence at court.
Mr Laffey said Roche has "quite significant problems" that need dealt with.
He added: "She is the only significant relationship he had in his life. There is a child of that relationship."
Mr Laffey said Roche acknowledges he needs to address his problems if he wants to have any future relationship with the child.