The boy's shocking death happened in a motel room in April 2010.
Police arrested Gabriela Espinosa hours later after an unusual chase on Highway 41.
Her attorney tells Action News she'll plead not guilty by reason of insanity.
Espinosa, now 25, hasn't denied she killed her baby boy, Gabriel.
But her attorney says she was insane at the time, and points to missed signs in the hours before the boy's death.
Police made an unthinkable discovery behind the door of this hotel room in April 2010: A 2-month-old boy left dead in his baby stroller, alone with two siblings, ages four and six.
Almost two years later, the little victim's mother sat in court, in tears, as a homicide detective told a judge about the confession she gave.
"She told me she had drowned her baby, but prior to drowning the baby, when she was in the motel room, she was holding her child and tried to suffocate it with a pillow," said Fresno Police homicide detective Ray Villalvazo.
Police say Gabriela Espinosa admitted she intended to kill all three of her children, but changed her mind when the youngest one struggled to survive.
"The baby was kicking and making too much noise, so she didn't want to physically abuse the older children because she thought her older child would fight her," Villalvazo said.
But it's what happened before the little boy's death on which Espinosa's attorney is focusing.
She says the new mother was suffering from post-partum depression and paranoia.
After a domestic violence incident with the children's father, Espinosa contacted social services.
The women's shelter at Marjaree Mason didn't have room for the family, so the new mom and her children were sent to the Economy Inn.
When they visited the nearby Manchester Mall's movie theater, staff called police on her because she was acting unusual.
Not long after, Denny's employees also called police on her, and an officer escorted the family back to the motel.
Her attorney says Espinosa offered to let the officer take her kids.
"This woman was in distress," said defense attorney Barbara O'Neill. "There was something mentally going on with her. She reached out to a lot of organizations asking for help."
Prosecutors argue Espinosa knew what she was doing and knew it was wrong.
A judge decided Tuesday there is enough evidence for Espinosa to stand trial for murder.