Fighting tears to tell a Suffolk jury about the morning her 43-day-old son died, a Ronkonkoma mother bitterly remembered the "false hope" that performing CPR on the little boy gave her.

Clarissa Hertzler, now 8 1/2 months pregnant with her next child, testified Monday against the baby's father, Christopher Foster, 32, of Kings Park. He is on trial in Riverhead, charged with the second-degree murder of Jonathan Hertzler.

During questioning by Assistant District Attorney Dana Brown, Hertzler explained why doing CPR until emergency workers arrived early on Oct. 11, 2011, felt so cruel -- even as, she said, Foster appeared impassive.

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"Every time you do it, it looked like he was breathing," she said, crying. "And Christopher just sat there, doing freaking nothing."

It was the same at Stony Brook University Hospital, after doctors could not revive Jonathan, Hertzler said.

"After a while, they [doctors] handed him to me and said, 'Do you want to say goodbye to your son?' " Hertzler said. Meanwhile, Foster fell asleep in the waiting room, she said.

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Although Hertzler said she had been concerned Foster handled the boy too carelessly, she didn't realize the true cause of his death until months later. That's when she learned an autopsy discovered a fractured skull, broken ribs and several bruises.

The baby had always been a source of friction in the relationship, Hertzler said, even before he was born. She said Foster, who had a 2-year-old son from another relationship, urged her to abort the pregnancy, and she almost did, until she heard the fetal heartbeat.

"He was angry," Hertzler said. "He said I was irrational. He said I was stupid."

The tension over the pregnancy caused them to break up, and they didn't get back together until Jonathan was three weeks old, she said. Even then, she said he refused to put Jonathan on his health insurance and would not sign the boy's birth certificate.

Defense attorney David Besso of Bay Shore objected, saying testimony about his client's attitude toward the baby had nothing to do with the murder charges.

"She's succeeded in depicting to the jury that he's not a very nice guy," Besso said.

State Supreme Court Justice William Condon agreed. "The charge here is murder," he told Brown. "Let's move on."

Testimony moved on to how the baby's crying aggravated Foster. Hertzler said he would often yell: "Why won't that baby shut . . . up?"

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And she said he was careless, once falling asleep with the baby on the couch. Foster explained a dime-sized bruise on the boy's temple by saying he bumped his head into a coffee table during feeding, she said.

Prosecutors said Hertzler couldn't continue Monday due to the physical and emotional strain. Besso will question her Tuesday.