Finishing his closing argument, lawyer Mark Geragos reminded jurors 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."
In a brief rebuttal, a prosecutor called the defense suggestion that that someone else abducted and killed Laci, then framed Peterson, simply "not reasonable."
"You can't base a reasonable doubt on an unreasonable interpretation of the evidence," prosecutor Rick Distaso told jurors. "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."
With closing arguments complete, jurors were to hear jury instructions and were expected to begin deliberations in the highly publicized case later Wednesday.
Prosecutors claim Peterson strangled or smothered his pregnant wife on Dec. 23 or 24, 2002, then dumped her weighted body into San Francisco Bay. Her body and that of the fetus washed ashore in April 2003. Geragos claims someone else abducted and killed the Modesto woman.
Peterson is charged with two counts of first-degree murder and could get the death penalty. The jury will also be allowed to consider second-degree murder, which does not require evidence of premeditation and carries 15 years to life in prison.
The trial began with jury selection in March, and opening statements were in June.
Geragos has argued the fetus was born well after Laci Peterson 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."
Prosecutors have argued the fetus wasn't as badly decomposed as Laci's body because it had remained in her womb for months before being expelled from her decaying body.
On Tuesday, Geragos conceded the former fertilizer salesman, who was having an affair at the time Laci vanished, is a liar and a cheat, but said he shouldn't be convicted of murder.
"You're not supposed to just decide this case on whether or not you like Scott Peterson," he told jurors.
Geragos accused prosecutors of waffling on their theory of the crime, first claiming Peterson's motive was his affair with Amber Frey, then raising financial issues and finally pointing to Peterson's desire to be free from marriage.
"Clearly, Amber was not the motive," Geragos said. "Nobody was going to kill Laci Peterson and her child for Amber Frey."
In their closing arguments, prosecutors made their case for premeditation, contending each bit of evidence is like a piece of a puzzle that convicts the former fertilizer salesman.
But Geragos said pieces are missing in that puzzle, such as the fact that Peterson paid a bill for Laci's health insurance the day before she vanished.
Another piece, he said, is the lack of physical evidence. "Maybe the logical explanation for the fact that we have no evidence of her struggling in that house, dying in that house, is because it didn't happen in that house."
He also tried to cast doubt on prosecutors' contention that Peterson had a financial motive for the killing, noting Laci was set to inherit nearly $1 million.
"She meant more to him alive than dead," Geragos said.