REDWOOD CITY, Calif. - Scott Peterson's defense began with a dramatic gambit - an assertion that Peterson could not have killed his pregnant wife because the fetus she carried was born alive.

Attorney Mark Geragos told jurors in a two-hour opening statement Wednesday that the boy whom the couple intended to name Conner didn't die in the womb, as prosecutors assert.

"If this baby was born alive, clearly Scott Peterson had nothing to do with this murder," Geragos said. "The evidence is going to show that (Laci) was alive on Dec. 24 when Scott went to the marina."

Geragos indicated he will call experts to testify that the fetus was older than it would have been if it died at the same time as Laci, and that the umbilical cord was cut in such a way that the child must have been removed from Laci Peterson while still living.

Prosecutors have said their experts will testify that the fetus was expelled well after Laci Peterson's corpse was dumped into San Francisco Bay.

Geragos' maneuver took some legal observers by surprise. Robert Talbot, a professor at the University of San Francisco School of Law, called it "dramatic, but dangerous."

"But if he can do it, it will go a long way to reasonable doubt," Talbot said.

Prosecutors allege Peterson killed his 8-months pregnant wife on or around Dec. 24, 2002, then disposed of her body during what he tried to disguise as a fishing trip.

Geragos' opening statement was the first formal glimpse inside a strategy to clear Peterson by suggesting that somebody else was the killer. Peterson, 31, could face the death penalty or life without parole if convicted in the trial that's expected to last six months.Prosecutors have portrayed Peterson as a lying cheat who killed his wife because he was having an affair with massage therapist Amber Frey - and because he wasn't ready to become a father.Geragos countered Wednesday that while the former fertilizer salesman had a mistress, it doesn't mean he killed his wife."He's not charged with having an affair. ... The fact of the matter is that this is a murder case and there has to be evidence," he said.Geragos downplayed Peterson's interest in Frey, saying they only went out on two dates, and characterized him as a giddily expectant father who accompanied his wife to all her doctor's appointments.During his opening statements Tuesday, prosecutor Rick Distaso didn't promise jurors they would hear about a murder weapon or an eyewitness to the crime, and Geragos seized on the circumstantial nature of the case.Authorities in the couple's hometown of Modesto secured more than 100 bags of material from Peterson's home, car and warehouse, and state crime lab scientists analyzed the evidence exhaustively, Geragos said."What did they get out of all those tests? Zip, nada, nothing," he said.Prosecutors said Peterson used a recently purchased boat to ferry his wife's body to San Francisco Bay from a marina in Berkeley. Her remains and that of the fetus washed ashore in April 2003, near where Peterson says he took the solo fishing trip the previous December.The day closed with testimony from the first prosecution witness - the Petersons' housekeeper, Margarita Nava. Nava testified in Spanish that she cleaned the home's kitchen floor Dec. 23.Prosecutors contend Peterson also cleaned and mopped his kitchen after killing his wife, and have seized the mop as evidence.