Peterson Baby At Heart Of Case

Scott Peterson's fate appears to have been sealed when the bodies of his pregnant wife and fetus washed up separately not far from where he says he was on a solo fishing trip in San Francisco Bay.

That alibi has been the most damning evidence against him in his murder trial.

Some legal experts say the defense could still win by proving one single fact: that the fetus was born alive long after Laci was reported missing. That would mean Peterson most likely couldn't have killed them because police watched his every move in the weeks after Laci disappeared on Christmas Eve 2002.

However, in testimony that included wrenching autopsy photos prosecution experts declared the baby died in Laci's womb, presenting a challenge for lead defense attorney Mark Geragos as the defense was to begin its case Tuesday.

In his opening statement, Geragos promised to prove that the baby Laci Peterson was carrying was born alive.

"He has made the promises to the jury," criminal defense attorney Chuck Smith said. "If he does not fulfill them the jury has reason not to trust him and if they don't trust him and trust the prosecution they are more likely to do what the prosecution asks and render a verdict of guilty."

Geragos almost certainly will not put Peterson himself on the witness stand, reports CBS News Correspondent John Blackstone. Jurors have already heard the defendant in television interviews and wiretapped phone calls both proclaiming his innocence and telling lies.

Geragos won't dispute that Peterson is a liar who cheated on his wife, but he will say that does not prove Peterson is a killer.

The experts' testimony left holes that defense attorneys hope to fill when they begin their case.

Dr. Brian Peterson, who performed the autopsy on the fetus, a boy the couple planned to name Conner, testified there was no evidence Laci had given birth before her death. But later, appearing to contradict himself, he said the fetus appeared to be full term.

Geragos implied the fetus could have been physically removed from a tear in the top of Laci's uterus, from where Dr. Peterson said it was eventually expelled.

"I couldn't say yes or no," Dr. Peterson replied.

Alison Galloway, an anthropology professor at University of California, Santa Cruz, estimated the fetus' age to be between 33 and 38 weeks based on bone measurements. Laci Peterson was believed to be 33 weeks' pregnant when she disappeared.

Geragos noted how inexact her estimate was, given the state of decomposition.

"I hate to say mushy, but that was sort of the way it was and that doesn't allow you to get an accurate measurement," Galloway said of the fetus.

Prosecutors allege Peterson killed his pregnant wife on or around Christmas Eve, then dumped her into the bay. The bodies washed up about four months later, mere miles from where Peterson claims to have been fishing alone the day his wife vanished.

Defense lawyers claim someone else abducted Laci and possibly held her captive while police homed in on Peterson, then crudely cut the fetus from her belly before framing her husband after learning of his widely publicized alibi.

"If this baby was born alive, clearly Scott Peterson had nothing to do with this murder," Geragos told jurors.

If Geragos can convince the jury the fetus was indeed at or near full term when it died, jurors likely couldn't convict Peterson for the killings. Laci's due date was roughly six weeks after her disappearance.

"He needs to at least show some doubt and some could argue that he's already done that with a concession from the prosecution witness that he can't rule out a live birth," said Court TV's Beth Karas on CBS News' The Early Show. But "if he can't do anything better than the prosecution witness, he may be in trouble."

Geragos is also likely to call or recall some police officers as witnesses, as well as his own expert witnesses.

"From the defense standpoint, calling experts is safe," said legal analyst Smith. "You can pretty much control what an expert is going to say."

Karas says it is unlikely that Geragos will put defendant Peterson on the witness stand, but Peterson can insist on testifying. That would be a mistake, she says.

"If Scott Peterson takes the stand, he's going to face a cross-examination by the prosecution that could be dangerous, despite the fact that so many bad things about him have already come out," she said.