Conor Callaghan then rang the victim from prison and made threats before warning her not to give evidence against him in court.
Callaghan carried out the attack at the home of the victim's friend in Leeds on March 8 this year.
Leeds Crown Court heard the couple were on a sofa together when they argued after he received a text message from another woman.
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Daniel Penman, prosecuting, said the woman pushed Callaghan away and he reacted by punching her in the face.
He continued to shout abuse at her before picking up her mobile phone and smashing it.
Callaghan kicked the woman as she tried to cover her stomach to protect the unborn child.
She suffered cuts and bruising to her face and head from the attack.
Callaghan left and returned to the couple's home later that day.
Her uncle was at the property and told Callaghan to leave.
Callaghan smashed the window of the uncle's car as he left the property.
The defendant was out of prison on licence at the time of the incident and was recalled to custody after he was arrested.
He rang the woman from prison on two occasions and threatened her.
In a separate incident on November 5 last year, Callaghan rode away from police on a scooter after being asked to stop on Britannia Road, Pudsey.
He was disqualified from driving at the time.
He has a previous conviction for robbery.
Michael Walsh, mitigating, said Callaghan accepted that he would be facing a lengthy custodial sentence.
Mr Walsh said Callaghan intended to address his offending behaviour while in custody and planned to be a good father when he is eventually released.
Callaghan was jailed for three years and three months.
He was also banned from driving for 31 months.
Judge Andrew Stubbs QC said: "The assault was a nasty and prolonged episode of domestic violence."