MODESTO, Calif. -- Amber Frey, the former mistress of Scott Peterson and a key witness in his prosecution for the murder of his pregnant wife, Laci, and the couple's unborn child, is also expecting a baby, according to a report Thursday by Fox News.

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Citing sources close to Frey, Fox News' Rita Crosby reported that the former massage therapist was four months pregnant and that Peterson was not the father of the child.

San Francisco Assistant District Attorney Jim Hammer said on KTVU's Mornings On 2 Thursday that he wasn't sure what impact Frey's pregnancy would have on the Peterson trial.

"It's going to turn some people off on the jury," Hammer said. "Although she is not technically on trial...As you saw in the preliminary hearing, (defense attorney) Mark Geragos is going to put everyone on trial."

The report said Frey has settled into a romance with a 43-year-old chiropractor from Fresno. She reportedly used to work with the doctor for almost two years before she met and fell in love with Peterson, who she said lured her into a relationship after telling her his wife had died.

The chiropractor and Frey, according to Fox News, began dating early in 2003. At the time, Frey was working with Modesto police -- secretly taping some 200 phone calls between she and Peterson while the search was going for his then missing wife. Sources told Fox that Frey and her new boyfriend were very much in love and talking about getting married.

Frey, who has never wed and is the single mother of a 2-year-old girl, has been in seclusion since early in the case. She reportedly is attending beauty school in Fresno.

A gag order issued by the judge hearing the case has forced Frey to only speak through her attorney, Gloria Allred. However, she was the focal point of the recently concluded preliminary hearing in the case. For days, it was rumored she would be called to the stand, but in the end prosecutors decided to tell Frey's story through the testimony of the investigators who worked with her.

Modesto Police Det. Al Brocchini told the court that Frey called police just days after Laci Peterson's Christmas Eve disappearance.

"She said she met Scott on November 20th," the detective told the court. "He (Peterson) said he wasn't married at the time. Later, about December 9th, she found out he was married and confronted him; and he had told her that he had lost his wife and that she was -- this would be the first holiday that he would be without his wife. She said that he was still calling her. This was the 30th (of December). He was still calling her."

"She said she'd heard from him on the 25th, 26th, 27th, and 28th; and he was still calling her, saying he was out of the country and he would be able to be with her more exclusively like January 25th."

Frey came forward in January and faced reporters at a jammed-packed Modesto news conference and echoed that testimony.

"First of all, I met him on Nov. 20," she told reporters at the time. "When I was introduced, I was told he was not married. Scott told me he wasn't married. We had a romantic relationship. When I discovered he was involved in the Laci Peterson case, I immediately contacted the Modesto Police Department."

"Although I could have sold the photos of Scott and I to the tabloids, I didn't want to jeopardize the investigation. I knew it (going to the police) was the right thing to do.

Peterson was ordered to stand trial for the murder of his wife and unborn son after 12 days of testimony at a preliminary hearing in early November. The bodies of Laci and Conner Peterson were recovered from the shores of the San Francisco Bay in April, just miles from where Scott Peterson had told authorities he was fishing on Christmas Eve day when Laci disappeared from the couple's Modesto home.

Peterson's next hearing is set for Dec. 3 and -- if he ultimately goes to trial and is convicted -- he could be sentenced to death.