CONCORD, N.H. – The state Supreme Court on Wednesday upheld the first-degree murder conviction of a Somersworth man who stabbed his pregnant ex-girlfriend to death in June 2004.

Lawyers for Anthony O’Leary had argued he killed Treasure Genaw of Berwick, Maine, in the heat of a fight and that he should have been convicted of manslaughter with provocation, not premeditated murder.

They said a Strafford County Superior Court judge erred when he instructed jurors they must first rule out first- and second-degree murder before considering a manslaughter conviction.

The court agreed the judge should have issued different instructions but said the error was harmless because, given the evidence, the judge did not even have to tell the jury it could consider manslaughter.

“The evidence of the defendant’s guilt of first-degree murder is of an overwhelming nature, and the jury charge was inconsequential in relation to the strength of the evidence of guilt of first-degree murder,” Associate Justice Gary Hicks wrote for the court.

O’Leary, 21, is serving life in prison without parole. The sentence for manslaughter is 15 to 30 years.

Genaw was 17 years old and three months pregnant with O’Leary’s child when he stabbed her repeatedly with a utility knife. He had asked her out for ice cream and a chance to talk. They ended up arguing, and after she tried to get the car keys, he put her in a headlock, according to court records.

She cut his fingers with a utility knife because she couldn’t breathe. After she dropped the knife, he grabbed it and stabbed her repeatedly in a jealous rage, even after she begged him for help, the court said. He later confessed to killing her and leaving her body off Route 236 in South Berwick, Maine.

In police interviews, O’Leary said the stabbing grew out of an argument.

“We were arguing and fighting about stupid stuff,” he said. “I don’t know what I’ve done, it happened so fast.

“It wasn’t supposed to go that far, I usually just walk away,” he added. “Something in me snapped.”

But prosecutors said the killing was deliberate: Genaw tried repeatedly to escape and begged for her life, but O’Leary told her, “It’s too late.”

Genaw was stabbed nine times – in the eye, throat, abdomen and chest.