Ashburton farm contractor Buick, 32, and the victim, who the Herald has chosen not to name, had been long-term friends and in an on-and-off relationship.
But when she fell pregnant with their first child, Buick said he didn't want her to keep the baby as he was seeing another Ashburton woman.
Late on the night of February 4, the victim texted Buick asking if he could visit her to discuss the baby, Christchurch District Court heard today as Buick admitted the savage attack.
They arranged a time and Buick arrived at her Christchurch home at about 1.30am.
He was armed with a baseball bat.
As soon as she opened her front door, Buick raised the bat above his head, and smashed it down across her head.
He struck her in the head and face "numerous times", according to the police summary of facts which described it as a "frenzied attack".
Buick then tried to hit the woman in the stomach with the bat, the court was told.
"The victim put herself into a foetal position in an attempt to protect her unborn child," police said.
"She has covered her head with her hands and has been struck in the head, hands and body with the baseball bat."
Buick hit her so hard that the bat snapped in half.
He then began kicking her around the head and body until she laid on the ground on her stomach "pretending to be dead".
Police say that Buick then fled the home without checking on the woman's condition.
The woman was hospitalised with a fractured skull, severe bruising to her face, in particular her right eye, and a broken eye socket. She also suffered a serious laceration to her forehead, severe bruising to the back of her right hand, and bruising to her right leg.
The police summary does not say whether the unborn child survived the assault.
The Herald has reached out to the victim, but she is yet to comment.
When police caught up with Buick, who has no previous criminal history, he declined to speak to them.
Judge Tony Couch gave Buick his first strike under the three strikes warning legislation and remanded him in custody for sentencing on April 12.
The judge called for home detention assessment reports but warned Buick that he could face jail time.