Updated

Nearly two months before Andrea Yates drowned her five children in the family bathtub, her two oldest sons rushed into the living room and asked their grandmother why their mom was filling the tub to the brim with water, the older woman testified Thursday.

Dora Yates, testifying for the defense in the second week of Andrea Yates' murder retrial, said she didn't know why her daughter-in-law filled the tub more than half full that day in May 2001.

"I asked her why ... and she said, 'because I might need it,"' Dora Yates testified.

Andrea Yates was convicted of capital murder in 2002, but her conviction was overturned by an appeals court because of erroneous testimony.

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In her retrial now under way, she again pleaded innocent by reason of insanity. Her attorneys have never disputed that she killed the youngsters but say she suffered from severe postpartum psychosis and didn't know her actions were wrong. Prosecutors argue that she knew what she was doing.

Dora Yates said she thought her daughter-in-law, who was in a psychiatric hospital for a couple of weeks shortly before the incident, had simply forgotten the bath water that day in May.

She said she never could have imagined she would hurt the children.

Dora Yates, who lives in Tennessee, had arrived in Houston in mid-April to help with 6-month-old Mary, 2-year-old Luke, 3-year-old Paul, 5-year-old John and 7-year-old Noah. She said her daughter-in-law seemed to improve after a second stay at the mental facility.

She never feared for the youngsters' safety, only that Andrea may have neglected something, she said.

But in the days leading up to the drownings, Andrea seemed to be slipping a bit, Dora Yates testified. She paced in circles around the house, scratched her head and stared at walls.

Then on June 20, 2001, Rusty — after getting a call from Andrea — called his mother and told her to go to the house, Dora Yates testified. When she arrived, she saw her son and police cars, and an officer told her that Andrea had killed the children.

"I screamed and screamed and screamed and screamed and screamed," Dora Yates testified. "I was just really a wreck."

Yates, 42, will be sentenced to life in prison if convicted. After the first jury rejected execution, prosecutors could not seek the death penalty again because they found no new evidence.

Prosecutors say that although Yates may be mentally ill, she does not meet the state's definition of insanity: when, because of a severe mental disease or defect, someone does not know at the time of a crime that one's actions were wrong.

They say Yates planned the drownings for when she'd be alone with the youngsters, after her husband went to work and before her mother-in-law arrived.