A Kings Park man who couldn't persuade his girlfriend to abort her pregnancy killed their son 43 days after he was born, a Suffolk prosecutor told jurors Monday.

But the defense attorney for Christopher Foster, 32, said there's no evidence of that. Baby Jonathan Hertzler just as easily could have been killed by the baby's schizophrenic grandmother, who was hospitalized after the Oct. 11, 2011, death when she said she wanted to kill people, defense attorney David Besso of Bay Shore said.

Foster, a former heavy equipment operator, is charged with second-degree murder and manslaughter in the baby's death. The trial began Monday, after state Supreme Court Justice William Condon denied a prosecution request to keep the grandmother's psychiatric history out of the trial.

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Besso said the information was important.

"I think the jury has a right to know who was in the house the night this baby died, and what her condition was," Besso said.

Condon also ruled that Besso can ask the baby's mother, Clarissa Hertzler, about invoking her right to not incriminate herself at a Family Court proceeding about the death.

Assistant District Attorney Dana Brown told jurors that Foster did not share Hertzler's joy about having a baby. When she refused to have an abortion, they broke up, she said.

But Foster reconciled with her and moved in to her Ronkonkoma apartment after he met his 3-week-old son at Smith Haven Mall, Brown said.

But he complained to his friends about the baby keeping him up at night. And then Hertzler noticed a bruise on her baby's head the night before he died, Brown said. Foster said he had accidentally knocked him into a nightstand, she said.

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That night, Foster was the last one to hold his son, at about 3 a.m. At 5:30 a.m., Brown said he called his boss and said the baby wasn't breathing. But Foster didn't wake Hertzler and didn't call 911, Brown said, waiting more than an hour to say anything.

"The baby suffered brutal injuries, horrible injuries," Brown told jurors. An autopsy found injuries to his head, abdomen, eyelids and mouth. His skull was fractured and four ribs were broken, she said. "His brain swelled, with nowhere for it to go."

Brown said Hertzler's mother had her schizophrenia under control then and loved her grandson. "She's not a violent lunatic who suddenly killed the baby," Brown said.

Besso told jurors to consider the psychiatric hospitalization of the baby's grandmother, along with Hertzler's initial insistence to police that "Chris doesn't have a violent bone in his body."

Besso also warned jurors not to be distracted by Foster's reluctance to have a baby with Hertzler, which he said is not the same as wanting the baby dead.