Sonia Hermosillo, 31, of La Habra was arrested Monday night, hours after the baby was thrown over a parking structure railing at Children's Hospital of Orange County, Orange police Sgt. Dan Adams said.
The infant remained in critical condition Tuesday at the University of California, Irvine, Medical Center, which has a trauma unit.
Hermosillo, who could make a first 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 was also on an immigration hold, he said.
A message left at a cell phone number associated with Hermosillo's address was not immediately returned Tuesday.
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.
"Our investigators did tell me that they're certain she's the suspect and we're not looking for any other suspects," Adams said.
A witness on the ground saw the baby falling through the air just after 6 p.m. Monday and called 911, authorities said.
Surveillance video showed Hermosillo's tan Chevrolet Blazer sport utility vehicle with an empty child seat leaving the parking structure a short time later, the sergeant said. The female driver was the only person in the SUV, and the license plate was traced to the Hermosillo home.
Adams said that during the investigation, the La Habra Police Department notified detectives that a man had reported his wife, Sonia Hermosillo, and their son missing.
An Orange police officer driving past Children's Hospital at 10:15 p.m. spotted the Blazer driving on a street about 100 yards from the crime scene and Hermosillo was at the wheel, Adams said.
Police said Tuesday that Hermosillo was cooperating with the investigation. They declined to release any further details.
Court records in Orange County indicate Hermosillo has no major criminal record but pleaded guilty to four traffic violations in La Habra in 2008, including driving without a valid license and having no proof of insurance.
It's unclear whether the family had any connection to Children's Hospital before the incident, Adams said. A hospital spokeswoman referred all calls to police.