Laci Peterson (search) stayed fit during her pregnancy by walking often and regularly attending yoga classes, her half sister testified Monday under defense cross-examination.

Scott Peterson (search), charged with killing his wife, has told police she was preparing to walk the couple's dog when he left for a solo fishing trip on the morning she disappeared, but prosecutors have maintained that she stopped walking the dog weeks earlier at her doctor's request after she suffered repeated dizzy spells.

During his questioning Monday, Peterson's attorney, Mark Geragos (search), portrayed Laci Peterson as a fit mother-to-be.

"She was very, very conscious of her weight ... and staying fit?" Geragos asked Laci Peterson's half sister, Amy Rocha (search).

"Yes," Rocha answered.

Prosecutors claim Scott Peterson killed his wife in their Modesto home on or around the morning of Dec. 24, 2002, because he was having an affair, then drove her body to San Francisco Bay and dumped it from his small boat.

Defense lawyers argue that someone abducted Laci Peterson while she was walking in a nearby park and then framed him.

The severely decomposed remains of Laci Peterson and her fetus ashore in April 2003, not far from the spot where Peterson claims to have been fishing. Authorities could not pinpoint exactly how or when they were killed.

Peterson, 31, could face the death penalty or life without parole if convicted. The trial is expected to last up to six months.

Geragos showed Rocha a picture of the Petersons standing on a beach in Carmel just a week before she vanished. He said the photo was taken nearly a mile away from the lodge where the couple stayed and that they had walked there.

"She was walking. She was active. She was fit, right?" Geragos asked.

"Yes," Rocha replied.

During Rocha's direct testimony last week, she said her half sister had not been feeling well and often became sick when she walked.

On Monday, Rocha said she had never seen the couple fight.

"In fact, the entire time you've known them they appeared to get along very well, right?" Geragos asked.

"Yes," Rocha said.

"As far as you knew, this was a marriage that was working and working very well, right?" Geragos asked.

"Yes," Rocha again replied.

Laci Peterson's mother, Sharon Rocha, was expected next on the stand.

Meanwhile, the father of Peterson's mistress, Amber Frey (search), said Monday he has been subpoenaed to testify in the trial. Ron Frey also said he had retained a defense attorney but would not say why.

Frey said he does not want to testify because he doesn't want to be restricted by a court-imposed gag order that bars all parties from talking about the case.

"I don't want to sit back and take it," Frey told The Associated Press.

There was no indication when Frey or his daughter will be called to testify because the witness list is sealed.