Amber Frey (search), the former girlfriend of accused murderer Scott Peterson (search), is four months pregnant, sources close to Frey told Fox News Wednesday.

The father is not Peterson, who has been imprisoned since his arrest in April, but Frey's 43-year-old chiropractor boyfriend from Fresno, Calif., sources told Fox.

Frey, 28, reportedly used to work with the doctor for almost two years before she met and fell in love with Peterson, the sources told Fox. She and the chiropractor were only friends until January, when they began dating, according to sources.

Frey has never wed, but is the single mother of a 2-year-old girl. She is said to be thrilled and in love, and the couple are talking about possibly getting married, sources told Fox. The former massage therapist is currently attending beauty school in Fresno.

Peterson's next hearing is set for Dec. 3. The trial date hasn't yet been set.

Frey is a key component in the case against Peterson, who is charged in the killings of his 27-year-old pregnant wife, Laci Peterson (search), and their unborn son, Conner.

Prosecutors are planning to argue that Peterson's relationship with and love for Frey were the motives for the murders.

Frey has been cooperating with law enforcement authorities, who wiretapped her phone and taped hundreds of calls Peterson made to her before and after she came forward with the news of the affair.

Frey has said Peterson never told her he was married but had eventually explained that he "lost his wife."

Peterson has maintained his innocence and has said the affair with Frey was purely physical. He said Laci knew about the affair and had accepted it.

Though there was much talk of Frey testifying during the preliminary hearing, which ended earlier this month, she was ultimately not called to the witness stand. Sources told Fox that she was warned four times that she'd be testifying but then was never asked to take the stand.

Frey is expected to testify at the trial, according to her attorney, Gloria Allred (search).