MOUNT CLEMENS, Mich. -

The mother of 2-year-old Damian Sutton was the first to testify on Thursday.

She told the courtroom about the day she will never forget -- when her boyfriend called her at work and told her young Damion had stopped breathing.

"My son was fine when I left so I knew right there something was wrong," said Nicole Sutton.

She wouldn't look at the man accused of killing her son. She only referred to Ronald Dimambro as the defendant.

Attorney: "Did you want to believe it was an accident?"

Nicole Sutton: "Yeah, because I couldn't picture somebody hurting my baby."

That's exactly what prosecutors allege happened in August. They say 27-year-old Dimambro shook the baby so violently that the toddler died six days later at Children's Hospital.

"We had to pull the plug because they said his brain was 80 percent gone. They tried surgery. Nothing, they couldn't help him," said Nicole Sutton.

Dimambro's lawyer said even though his client admits to shaking the child, an earlier head injury also contributed to Damian's death. While unsupervised, the child fell off a barstool.

"They hear a big thud. It's a thud. This is a kitchen floor," said defense attorney Randy Rodnick.

However, medical experts say this was no accident and Damian's injuries were intentionally inflicted.

"Damian was abused. I think this is child abuse," said Dr. Mary Lu Angelilli, of Children's Hospital.

Dimambro was babysitting the day EMS were called to the Washington Township home while Nicole Sutton was at work.

"I gave Damion a kiss goodbye and he said, 'I love you mommy,' ... so that's the last time I had seen my baby boy," she said.

The prosecution says Dimambro changed his story a number of times while being questioned by police, including how long he shook the boy to get him to stop crying.

The trial is expected to last until next week.