Andrea Yates was suffering from severe psychosis, while still feeling strong maternal love, when she drowned her five small children in the bathtub, defense attorney George Parnham said as the Houston mother's capital murder trial began Monday morning.

"Post-partum depression is the cruelest and most severe of mental illnesses," he told the 8-woman, 4-man jury. "It takes the very nature of motherhood and changes the reality."

Yates has entered a plea of not guilty by reason of insanity, a defense rarely used and even more rarely successful in contested cases.

The prosecution countered that though Yates is indeed mentally ill, the fact that she called police and told them what she'd done showed she knew right from wrong and was thus not legally insane.

"She told him [the detective] that she killed her children," said prosecutor Joe Owmby.

Opening arguments started Monday morning in the highly-publicized, long-awaited murder trial of Yates, 37.

Fox News Channel is covering the trial, expected to last about three weeks, live.

Yates, diagnosed as severely mentally ill and at times suicidal, drowned her five children ages 6 months to 7 years in the family bathtub June 20, 2001 and then called police and confessed to the murders.

Yates' attorneys say the former nurse can't be held accountable for her actions because she was so ill she thought she was doing what was best for her children — whom she was convinced had been damaged in their development because of her bad mothering.

"She did not appreciate the wrongness of her act and believed it to be right," Parnham said.

Yates has been diagnosed as psychotic, and her family has pointed to evidence of a history of post-partum depression and the possibility that she believed she was possessed by the devil when she killed her children.

"It can either be that there is something wrong with her, or that she is a really bad person," an unidentified woman told lawyers last week before being selected as one of the 12 jurors. "I don't know what the problem is."

Yet legal experts say Yates faces an uphill battle with an insanity defense. Not only is the legal move notoriously unreliable in real-life courtrooms — it works in less than one percent of trials — but Yates' most sympathy-provoking action may ironically harm her own case.

"The fact that she called the police right afterward and reported herself in essence really undermines the fact that she thought what she was doing was right," Baylor University law professor Brian Serr said.

It will likely be impossible for Yates' lawyers to make the jurors ignore the horror of her act.

"When you have a crime like this that is so heinous, I think the jurors' inclinations are likely going to be somewhat disinclined to find insanity," Serr said. "The fact that she was feeling psychological or mental pressure to kill them does not mean she was in some sort of psychotic state or that she — in a twisted fashion — perceived it to be right."

Jurors will hear the details of the case, including the emergency call Yates placed after she drowned the last child, Noah, 7. His body was discovered face down in a bathtub half full of water. The other dead children had been tucked into bed.

They also will hear the confession Yates gave to police when they arrived at her door, a description of the officers' discovery of the other four children's bodies and a taped interview that followed her arrest.

"We know that drowning children is wrong," Parnham said during jury selection. "Objectively, we could all sit here and say those actions are wrong, but you're going to be asked to view those actions through her eyes."

Prosecutors will likely point to testimony from Yates' competency hearing — that she had made the decision the night before, and that after her husband left for work she drowned her children one by one before her mother-in-law was to arrive.

"All of this indicates this wasn't a spur-of-the-moment act," trial consultant Stacy Schreiber said. "But again, it goes back to explaining the nature of mental illness and a person's fight to stay in control."

On the defense side, Yates' husband said she suffered from depression after the births of her two youngest children. Medical records detail her bouts with depression, showing that she attempted suicide twice after the birth of her fourth child in 1999 and was warned by a doctor to carefully consider whether she should have any more children.

Among the experts on the case are two of the nation's top forensic psychiatrists, whose experience includes serial killer Jeffrey Dahmer, Unabomber Ted Kaczynski and Susan Smith, who drowned her own two children by rolling her car into a lake.

Phillip Resnick, a psychiatry professor at Case Western University's School of Medicine in Ohio, is working for the defense. Park Dietz, who runs a California-based private forensic consulting firm, is testifying for the prosecution.

Yates faces two counts of capital murder. One is for the deaths of Noah and 5-year-old John. The other is for 6-month-old Mary. Texas law considers a murder a capital offense if more than one person is killed or if the victim is under age 6.

Charges are pending in the deaths of Paul, 3, and Luke, 2. Texas prosecutors typically forgo multiple capital-murder charges since only one conviction is generally needed for the maximum penalty.

A person found innocent by reason of insanity in Texas may be committed to a mental institution, then faces a series of hearings until the court releases the person from its jurisdiction, which can last as long as the jail time the defendant faced.

If Yates is found guilty, jurors would have to determine whether to sentence her to life in prison, or to death.

Serr believes jurors will opt for a punishment harsher than death — life.

"Given the nature of this crime, it might be a worse punishment for this woman to be locked up forever and to have to think day, after day, after day that she killed her children and they were perfectly aware of who had become their enemy," he said.

The Associated Press contributed to this report.