A jealous pregnant woman who hurled a knife at her partner that embedded in his forehead has today been sentenced to four months' home detention and 150 hours of community work.

Shontelle Vinta Marie Prasad, 21, had gone to pick up her drunken partner Mark Burton who had been socialising at a mate's place on August 2 this year.

On the way back to their home in the Christchurch suburb of Aranui, they began arguing.

Mr Burton pushed her face against the car window, causing bruising, a court heard.

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When they got home, he went to sleep on the couch in the lounge.

While he was asleep, Prasad took his cellphone out of his pocket and started rifling through his texts.

She found messages to a woman she did not know and phoned to confront her.

The mystery woman did not answer, and so Prasad turned her attention to Mr Burton.

They argued and Prasad furiously picked up a 30cm knife from nearby.

Christchurch District Court earlier heard that just after midnight Prasad threw the knife at him, which struck him in the head, and lodged in his skull.

Prasad phoned an ambulance and Mr Burton was taken to hospital where the knife was removed.

He was discharged later that morning.

When asked by police what happened, Prasad admitted "being angry when she threw it".

Prasad pleaded guilty to an amended charge of causing grievous bodily harm with reckless disregard for the safety of others.

The court heard that the pair began a relationship early last year and that at the time of the incident she was pregnant with their child.

She gave birth to the baby nine weeks ago.

Defence counsel Serena Bailey today said Prasad was "very keen"to put the incident behind her.

The couple have since split up but are jointly caring for their child.

Ms Bailey said Prasad had "snapped"when she read the text messages.

During the subsequent argument she had picked up the nearest item to her -- which happened to be a knife -- and had thrown it at him, Ms Bailey said.

Prasad, a mother-of-two, is "extremely remorseful", the lawyer said, with no previous convictions.

"The chances of hitting someone directly in forehead are ... remote," Ms Bailey said.

Mr Burton has had no ongoing consequences from the injury, the court heard.

Prasad has since undergone counselling, including anger management training, as well as engaging in the Early Start programme designed to help her parenting skills.

Judge Brian Callaghan referred to the victim impact statement where Mr Burton just said that he wanted her home to care for their baby.

Prasad was "emotionally overawed" for a number of reasons that night, Judge Callaghan said, including being assaulted by Mr Burton earlier in the evening.

"Whatever going on in that relationship at that point wasn't positive and the issue of the other female's cellphone number being on his phone ... seemed to be the straw that broke the camel's back," the judge said.

He sentenced Prasad to four months' home detention plus direction to carry out 150 hours of community work.