Jurors began deliberating the fate of Scott Peterson (search) on Wednesday, more than five months after testimony began in the murder of his wife and her fetus.

Judge Alfred A. Delucchi sent the panelists off with lunch into the jury room after about 45 minutes of instructions. They met for four hours before retiring for the day.

The jury must decide whether Peterson killed his pregnant wife and dumped her body in San Francisco Bay, or was merely a straying husband who was framed. The judge plans to keep the jury sequestered until it reaches a verdict.

In a brief rebuttal to the defense closing arguments, prosecutor Rick Distaso advised jurors not to find reasonable doubt in an unreasonable interpretation of evidence.

"It's just not reasonable that anyone put that body in the bay to frame him. If it's not reasonable, you must reject it," Distaso said.

Winding up their case earlier in the day, defense lawyers lashed out at the notion that Laci Peterson's fetus died in her womb. Lawyer Mark Geragos (search) reminded jurors that authorities never found the placenta or the fetus' umbilical cord, leaving little evidence to determine whether the male fetus was born alive and killed later.

If the fetus died later, Geragos said, "it's not Scott Peterson who did that."

Prosecutors claim Peterson, a fertilizer salesman who was having an affair, strangled or smothered his 27-year-old wife on Dec. 23 or 24, 2002, then dumped her weighted body into the bay. Her badly decomposed body and that of the fetus washed ashore four months later. Geragos claims someone else abducted and killed the Modesto substitute teacher.

Jurors have two choices should they decide to convict — first-degree murder, carrying a possible death sentence or life without parole, and second-degree murder, carrying two sentences of 15 years to life.

"First-degree murder you need two things, expressed malice and intent to kill and premeditation," Delucchi told jurors. Second degree, he said, means Peterson killed them but didn't plan it. The judge added the lesser charge after finding there was ample evidence to support a case that did not involve premeditation.

The trial began with jury selection in March, followed by opening statements in June.

Geragos has argued the fetus was born well after Laci Peterson (search) vanished, proving his client couldn't be the killer given the intense police surveillance of him in the days and weeks after she disappeared.

A prosecution witness testified the fetus likely died around the same time Laci was reported missing. A defense witness countered that the fetus could have been born weeks later.

"Was that baby wrapped in some kind of plastic? ... We don't know," Geragos told jurors. "The fact of the matter is, though, that that baby looks like it had something wrapped around it to protect it."

In their closing arguments, prosecutors made their case for premeditation, contending each bit of evidence is like a piece of a puzzle that convicts Peterson. Geragos countered that with so many missing pieces, jurors must decide there is too much reasonable doubt to convict.

"It's either a frame-up or he did it," said Robert Talbot, a professor at the University of San Francisco School of Law, who has been observing the trial. "And the frame-up just doesn't make any sense at all."