However, Richard 'Rickie' Joseph Fullerton was cleared of the more serious charge of assaulting her with intent to procure a miscarriage after the prosecution offered no further evidence on the count.
Antrim Crown Court Judge Desmond Marrinan QC told Fullerton that while what happened between them was possibly a mistake, Fullerton had certainly "made a mistake in assaulting a young girl when pregnant which I take to be a very serious matter".
The judge also warned Fullerton, with an address in Leckagh Drive, Magherafelt, that when it came to his pre-sentence report he explained fully his attitude and that he "hoped it is one of remorse".
However, Judge Marrinan said, the fact that the "child was born healthy and well is the most important matter in this case".
Following Fullerton's conviction for attacking her while pregnant, his former girlfriend said she was "happy it has now come to an end".
The young woman, who did not want to be named, said that is all she wanted so that she and her family "can put this behind us and get on with our lives".
Initially Fullerton went on trial denying two charges, common assault between December 2009 and 1 February, 2010 and, secondly, with intent to procure the miscarriage of a woman, unlawfully used an instrument, or other means, namely a blow to her stomach.
However, after a taped police interview from the girl was played to the court, defence QC John Kearney asked for Fullerton to be re-arraigned on the assault charge.
Following his plea of guilty, prosecutor Robin Steer told the jury and Judge Marrinan that they would be offering no further evidence on the second count.
In the circumstances the judge directed the jury find him guilty by confession on count one, but to acquit him of the more serious second count.
Judge Marrinan said that the case had reached a delicate moment, before the end of the evidence of the girl, and that certain matters had to be explored.
In the earlier taped interview, the then teenage girl said that after the attack she thought it was "really sad" as she did not think he would do anything like that.
Asked why she had been punched, she said it was because Fullerton "didn't want me to have the baby".
The teenager, also revealed that in the following weeks, her fears proved only too true.
In several text messages, Fullerton gave her an ultimatum, either him or their unborn child.
He encouraged her to slip off to England for a supposed holiday, but in reality to seek an abortion.
Her horrified friends told her to end the relationship.
While she continued the relationship for a time, she refused to go to England.
"I didn't want to go to England to have an abortion. I remember people saying if you ever have an abortion you will regret it," she said in her taped interview.
She told police that she when initially she told him that a pregnancy test "had come back positive", a "very unset and annoyed" Fullerton said the test may have been "mixed up".
However, when she "turned round to give him a hug he punched me in the stomach twice ..... full force punch ... just below the belly button".
She said as she "sat on the bed doubled on two" her younger brother came in asking what was wrong, only for Fullerton to tell him that she had "a sore stomach", while she herself "felt sick".
Prosecution lawyer Mr Steer also told the court that the relationship between them began to falter and that Fullerton even confessed to the girl's aunt and grandmother after they both issued a warning to him not to touch her.
Mr Steer claimed that Fullerton, described as being "in a temper" when given the warning, told them the only time he had "lifted his hand" to her was when she first told him she was pregnant.
Mr Steer said it was after this that a social worker was informed before the police were later called in.
Fullerton will be sentenced next month.