A witness critical to Scott Peterson's (search) defense conceded on the stand that he made an assumption in calculating when Laci Peterson's (search) fetus died, a potential victory for prosecutors as Peterson's murder trial is nearing its conclusion.

The age of the fetus is crucial because defense lawyers maintain it was born alive, proving Scott Peterson couldn't have killed his wife -- given her due date of Feb. 10, nearly seven weeks after she vanished. By that time, a nationwide search was under way and Scott Peterson was under police observation.

On Thursday, Dr. Charles March (search), a gynecologist, 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.

The coroner who performed the autopsy on the fetus estimated its age at death to be about nine months, or full term. A prosecution witness said the fetus probably died between Dec. 21 and Dec. 24.

Defense lawyer Mark Geragos 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.

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."

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.