A mother accused of nearly killing her 7-month-old son by tossing him from the fourth floor of a hospital parking garage was treated for depression after the child's birth, her husband said.

Sonia Hermosillo, 31, was arrested Monday night, hours after the baby was thrown over a parking structure at Children's Hospital of Orange County, police Sgt. Dan Adams said.

The woman's husband, Noe Medina, told the Orange County Register that his wife had been hospitalized for postpartum depression in June after she said she didn't want their son, Noe Jr, who was diagnosed with congenital muscular torticollis — a twisting of the neck to one side.

The infant also wore a helmet to help correct his plagiocephaly, also known as flat-head syndrome, the Register reported. It was unknown if the boy was wearing a helmet when he was tossed over the railing.

"She didn't look at our son as normal," Medina said. "She didn't accept him. She didn't accept that he was like this."

Noe said the boy was receiving physical therapy twice a week and was showing signs of improvement.

Medina said his wife seemed better in recent weeks and he "began to trust her."

The infant remained in critical condition at the University of California, Irvine, Medical Center, which has a trauma unit.

Hermosillo, who could make a court appearance as early as Wednesday, was being held without bail for investigation of attempted murder and is undergoing a psychological evaluation in the jail's medical ward, said Orange County sheriff's spokesman Jim Amormino. She also was on an immigration hold, he said.

At a similar apartment complex less than two blocks away in a working-class neighbourhood of single-story homes and 1960s-style apartment buildings, former neighbor Carmen Ruiz said she saw nothing wrong with Hermosillo. They spoke almost every day, she said, and Hermosillo was a stay-at-home mother to her two young daughters while her husband worked in construction.

The couple, both immigrants from Mexico City, didn't seem to fight and were friendly with neighbours, she said.

Police have not released a motive and the Orange County district attorney's office declined to comment until prosecutors there received the case from detectives, said Farrah Emami, district attorney spokeswoman.