Associated Press In this television image, Andrea Yates listens as opening statements are given during her trial in Houston yesterday. Yates faces two capital murder charges in the drowning deaths of three of her five children.

HOUSTON - A lawyer for Andrea Yates said yesterday that the Houston mother was suffering from postpartum depression and severe psychosis, "the cruelest and most severe of mental illnesses," when she drowned her five children.

George Parnham told the jury in opening statements that Yates' mental illness kept her from knowing right from wrong.

"It takes the very nature and essence of motherhood - to nurture, to protect and to love - and changes the reality," he said.

The 37-year-old woman faces two capital murder charges in the June 20 deaths of three of her five children, ranging in age from 7 years to 6 months. Defense attorneys say the former nurse turned stay-at-home mom is innocent by reason of insanity.

Prosecutor Joe Owmby told jurors yesterday that under Texas law the defense must prove she is insane.

"She is presumed to be sane, to know right from wrong," Owmby said. "The state bears no burden of proof to prove she was sane."

Legal experts say the defense could face a difficult job during the trial, which is expected to last three weeks.

"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," Baylor University law professor Brian 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."

"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."

Before jurors get to hear evidence about Yates' mental state at the time of the drownings, they will hear the details of the case, including the 911 call Yates placed after she drowned the last child, Noah, 7, whose body was discovered face down in a bathtub half full of water.

They also will hear the confession Yates gave to police when they arrived at her door, how the officers found the youngest four children's wet bodies on a bed covered with a sheet, and a taped interview that followed her arrest.

Prosecutors also will likely point to testimony from Yates' competency hearing that she made the decision to drown her children the night before, and that after her husband left for work she drowned her children one at a time 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."

Yates' husband said she suffered from depression after the births of her two youngest children. Medical records detail her bouts with depression, and show 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.

The doctor who gave that warning is scheduled to testify, along with other medical experts, police officers, detectives, friends and relatives.

Among the experts on the case are two of the nation's top forensic psychiatrists, whose experience includes serial killer Jeffrey Dahmer, Unabomber Ted Kaczynsky, and Susan Smith, who drowned her 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.

"They are both brilliant and gifted and will do the best possible job for their respected sides," said Neil Kaye, a fellow psychiatrist and friend of both.

"Phil would not take the defense side of this case unless he really believed it," Kaye said.

Dietz makes the complicated simple and juries often embrace simplicity, said lawyer David Bruck, who hired Resnick for Smith's 1994 murder case, knowing that Resnick was an expert on mothers who kill their children - and on Dietz.

"We desperately need to make sense of things like this and Dietz is very good at making sense of things," Bruck said.

One of the capital murder charges is for the deaths of Noah and 5-year-old John. The other is for 6-month-old Mary. Texas law considers 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 face 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."