Andrea Yates, the suburban Houston woman who confessed to drowning her five children one-by-one in a bathtub last week, is "in a very deep psychosis," her lawyer said Monday.

Attorney George Parnham said he has met with psychiatrists treating the 36-year-old Yates at the Harris County Jail, where she is on a 24-hour suicide watch. She has been charged with one count of capital murder and could receive the death sentence if convicted.

Parnham said he expects Yates to plead not guilty because of insanity.

"My observation is that she is still in a very deep psychosis," he told CBS' The Early Show. "We are having her treated and examined by very professional mental health experts who care deeply for their patients."

He said the medication she is on likely will kick in "and there will be some ability to have a rational conversation with her," Parnham told ABC's Good Morning America. "That moment in time has not yet arrived."

Yates has been charged with one count of capital murder in the deaths of her sons Noah, 7, and John, 5. The other children found dead in the family home were Paul, 3; Luke, 2; and 6-month-old Mary. Authorities said other charges might follow.

Parnham said he is gathering background on Yates' mental health history before making a final decision on her defense.

"I've accumulated evidence in the last 24 hours that strongly suggests that the mental status of my client will be the issue, which means entering a not-guilty plea by reason of insanity," the lawyer told the Houston Chronicle.

He would not say what evidence indicated Yates was insane when she killed the children. However, the lawyer said he does not think the case warrants the death penalty and he hopes prosecutors decide against seeking it.

Yates' husband, Russell, has said that his wife's postpartum depression, coupled with her father's recent death, drove her to harm their children.

George Dix, a law professor at the University of Texas, said insanity defenses are rarely used and rarely succeed when they are. To be found innocent by reason of insanity, Yates would have to show that she was so mentally impaired that she couldn't see circumstances for what they were.

For example, if she "believed her children to be devils, she's entitled to acquittal," Dix told The Dallas Morning News in Sunday's editions.

Michelle Oberman, a law professor at DePaul University in Chicago and author of the upcoming book Mothers Who Kill Their Children, said postpartum depression has been a defense element since the mid-1980s.

"The best chance she has, if she's got any good chance, lies in hoping the jury understands the circumstances under which she was operating and to understand the reality of postpartum depression," Oberman said.

About 10 percent to 20 percent of mothers face some level of postpartum depression. Most cases are mild, and can be treated with counseling and medicine. On rare occasions, post-partum depression has been known to cause psychosis or delusions.

The Associated Press contributed to this report.