Prosecutors Thursday attacked a witness critical to Scott Peterson's (search) defense who nervously conceded he made an assumption as he calculated when Laci Peterson's (search) fetus died.

Dr. Charles March, a gynecologist asked by the defense to examine prosecution witnesses' findings and Laci Peterson's medical records, testified that based on bone measurements of the dead fetus and reviewing ultrasounds taken of Laci, the fetus probably died on Dec. 29, 2002, at the earliest, five days after the pregnant schoolteacher vanished.

That would undercut the prosecution's claim that Scott Peterson murdered his wife on or around Dec. 24, then dumped her body into San Francisco Bay. But March also said he based his findings, in part, on anecdotal evidence of when Laci may have discovered she was pregnant.

According to previous testimony, Laci Peterson told one of her friends on June 9, 2002, that she was pregnant.

Under cross-examination, March acknowledged he inferred from that information that Laci had just found out she was pregnant based on a home test June 9 because, he said, Laci would likely have told her friend about it immediately.

"Where in the medical records does it talk about Laci Peterson using a pregnancy test on June 9?" prosecutor Dave Harris asked.

"Nowhere," March replied, becoming obviously flustered, shifting nervously in his seat and biting his lower lip.

"So you're making an assumption to form a medical opinion, isn't that correct?" Harris prodded.

"Based on 30 years of being a doctor ... that's a pretty good assumption," March said.

Defense lawyer Mark Geragos (search) promised jurors during his opening statement that he would prove the fetus died after Laci vanished. Legal experts agreed March's testimony fell tremendously short of delivering on that promise.

"This was meant to be one of the high points of the defense and it just sunk," said James Hammer, a former prosecutor and trial observer.

Robert Talbot, a professor at the University of San Francisco School of Law, called it "devastating."

"How can you base a scientific medical opinion on something that came up in conversation without anything to back it up? You can't come into a court of law and base a key fact on that and expect to have credibility," Talbot said.

Later, however, on redirect, Geragos noted that Laci had told several friends of her pregnancy on that June day. It is expected the defense will call witnesses next week to testify that Laci also told them she learned of her pregnancy on June 9.

Geragos then asked March if his findings would have been different without that information.

"Not really," March said. "I think it's nice to have that information because it reinforces."

Hammer said Geragos may be able to save March's credibility if he "can definitively prove when she took the test with admissible testimony."

The age of the fetus is crucial because prosecutors claim it was expelled dead from Laci's corpse. Defense lawyers maintain it was born alive, proving Scott Peterson couldn't be the killer given its due date of Feb. 10, nearly seven weeks after Laci vanished. Experts agree proving this fact was crucial to the defense case.

The coroner who performed the autopsy on the fetus estimated its age at death to be about nine months, or full term. A forensic anthropologist testified she calculated the fetus' age at between 33 weeks and 38 weeks at death.

Another prosecution witness said the fetus probably died between Dec. 21 and Dec. 24.

The remains of Laci Peterson and her fetus washed up about four months after she disappeared, and a few miles from where Peterson claims to have been fishing alone the day his wife vanished.

Defense lawyers claim someone else abducted Laci while she walked the couple's dog and killed her, then framed her husband.

Judge Alfred A. Delucchi told jurors Thursday they would be sequestered for deliberations set to begin Nov. 3. They have been free to go home throughout the duration of the five-month old trial.