SANTA ANA, Calif. - A 32-year-old La Habra woman accused of dropping her 7-month-old son to his death from the fourth level of a parking structure at Children's Hospital of Orange County pleaded not guilty by reason of insanity.

Sonia Hermosillo also pleaded not guilty to murder and child assault causing death during her court appearance Wednesday.

Hermosillo will return to court Feb. 22 for a hearing on a motion to dismiss charges and to receive a report from two doctors appointed to evaluate the defendant, defense attorney Jacqueline Goodman said. A judge may also set a trial date at that hearing.

Hermosillo's trial will be in two phases. In the first phase, the jury will consider whether she committed a crime. If she is convicted, the jury will then consider whether she was so mentally ill at the time that she did not understand what she was doing was illegal or wrong.

If Hermosillo is found guilty by reason of insanity she would be sent to a state mental hospital instead of a prison.

Hermosillo was suffering from postpartum psychosis when her son, Noe Medina, was dropped to his death from the parking structure Aug. 22, 2011, Goodman said.

"It's sort of akin to someone sleepwalking," Goodman said of Hermosillo's state of mind at the time.

Deputy District Attorney Scott Simmons, however, said there's evidence Hermosillo knew what she was doing.

Simmons said Hermosillo drove to the top of the hospital's four-story parking structure, put the boy on the ledge and pushed him off, then drove back down and had her parking ticket validated.

The defendant is also accused of taking a protective helmet off the baby -- who was born with flat-head and twisted-neck syndrome -- before dropping him some 40 feet to his death, Simmons said.

Hermosillo drove around the area for hours before she was arrested near the hospital, Simmons said.

Hermosillo was bound over for trial after her preliminary hearing Dec. 11.

Goodman challenged the assertion that the parking ticket validation showed evidence of Hermosillo understanding what she was doing.

"The parking ticket validation to me is evidence of the contrary -- that she doesn't understand the wrongfulness of her actions," Goodman said. "I don't see that necessarily as evidence of planning, if it's evidence of anything."