HOUSTON, Texas (CNN) —The husband of a Texas woman on trial in the drowning deaths of their five children could take the stand Wednesday to help bolster the defense's claim that the defendant was insane when she drowned her five children

Russell Yates is expected to testify about his wife Andrea Yates' struggles with mental illness in the years before the killings. Andrea Yates' brother and mother also are expected to testify for the defense.

Andrea Yates, 37, is charged with two counts of capital murder in the deaths of three of the children -- Noah, 7, John, 5, and Mary, 6 months. She is not on trial for the drownings of Luke, 3, and Paul, 2.

The Houston-area mother confessed to drowning the children, but she has pleaded not guilty by reason of insanity.

Testimony in the trial, which in its second week, is scheduled to resume at 11:30 a.m. EST Wednesday.

On Tuesday night, Russell Yates told reporters he was nervous about his testimony, but he declined to give details of what he would say.

Attorneys for Andrea Yates have called a variety of medical experts in an attempt to show that the defendant was insane at the time of the killings.

Dr. George Ringholz of the Baylor College of Medicine testified that Yates was schizophrenic and suffered psychosis the day she drowned her five children. Ringholz conducted an extensive evaluation of Yates based on her medical history and interviews with three family members, including her husband.

Called by the defense, Ringholz said that Yates had stopped taking her anti-psychotic medication June 6, two weeks before the killings. He said the drug would have been completely out of her system by the time of the June 20 drownings, leading to a dramatic increase in her symptoms.

"There's no doubt in my mind that Andrea Yates suffered from psychosis on June 20," Ringholz testified.

Earlier this week, a psychiatrist under cross-examination by the prosecution said that Yates thought about killing her children the night before she carried out the act.

Dr. Melissa Ferguson, a psychiatrist at the Harris County Jail, testified that Yates told her about her thoughts in an interview after she was arrested.

Prosecutors are trying to show that Yates knew what she was doing when she killed her children and that the killings were premeditated acts.

"Andrea Yates said she had thought about drowning the children the night before," said Ferguson, describing a conversation with Yates in a lengthy six-day interview process.

Ferguson also told the court that Yates said that drowning the children was "the right thing to do."

"The children were going to be tormented the rest of their lives, and they were going to perish in the fires of hell," Ferguson quoted Yates as saying.

Last week, Ferguson had described Yates as "one of the sickest patients I had ever seen." She said Yates exhibited signs of paranoia and delusions — that she told her, "I am Satan," and wanted to shave her head to reveal the "mark of the beast," or 666, that she believed was on her scalp.

Ferguson said Yates is now taking a cocktail of four medications: the powerful anti-psychotic medication Haldol; Cogentin, to counter the side-effects of Haldol; and the anti-depressants Effexor and Wellbutrin.

The prosecution will get a chance to rebut the defense's case. Prosecutors are expected to call to the stand Dr. Parke Dietz, a University of California-Los Angeles psychiatrist famous for convincing a Wisconsin jury that serial killer Jeffrey Dahmer was not insane.

Prosecutors rested their case Friday, one day after presenting their most dramatic evidence: a videotape and photographs taken shortly after police discovered the children's bodies and an audiotaped statement that Yates gave to police.