A woman who struck a pregnant love rival in the stomach has had six months added to her prison term.

Danielle Jordyn Tu Whangai, 24, is currently serving a sentence of two years one month imposed last year over another violent incident.

Yesterday, in the Dunedin District Court, that sentence was increased when the defendant pleaded guilty to assault with intent to injure and intimidation from an incident on February 15 last year.

A month before the attack she had been to see the victim - a woman she believed her partner had impregnated.

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Tu Whangai returned to the St Clair home with her boyfriend one afternoon to again confront the woman.

She raised the same concerns, "swearing at the victim angrily and calling her offensive names''.

The victim denied she was pregnant.

The court yesterday heard the woman was in fact expecting a baby at the time, though the identity of the father was unconfirmed.

Tu Whangai and her partner returned to their car after the spat.

"Not being a person to be intimidated'', the victim followed, yelling at the man to sort out the situation.

As the victim turned back for her house, Tu Whangai grabbed her by the T-shirt.

She broke away but the defendant lashed out again, pulling one of the woman's earrings out.

Tu Whangai clawed at the victim's face and wrenched her over by the hair.

"Holding the victim bent over, the defendant kneed her in the stomach twice while verbally threatening to cause harm to the baby,'' a police summary said.

Tu Whangai's partner managed to pull her off the victim and they drove off.

Judge Michael Crosbie said the woman had found the episode "really stressful'' and remained concerned about the defendant's attitude towards her.

She went to the hospital with stomach cramps but was discharged without incident.

Tu Whangai, who appeared in court by audiovisual link, was reportedly doing well in prison.

She said she had been declined parole in October so that she could undertake the Kowhiritanga programme alongside one-on-one counselling.

Kowhiritanga is a group-based programme for female offenders targeting "the attitudes and behaviours that contributed to their offending and teaching skills and new ways of thinking''.

Tu Whangai's sentence end date is now September 2020.

She will see the Parole Board before then.