The Scott Peterson murder trial, which began last week, promises to contain long lessons in forensics. Defense attorney Mark Geragos has pledged to show jurors that Peterson's wife, Laci, and her unborn son, Conner, survived weeks after cops believe Peterson killed them. Prosecutors maintain that Peterson, 31, murdered his pregnant wife on Dec. 23 or 24, 2002, then sank her body in San Francisco Bay. To counter this timeline, Geragos will cite autopsy reports showing that Conner was older than the 33 weeks he was thought to be when Laci disappeared. (The 33-week assessment comes from Laci's obstetrician, who based the estimate on a three-month-old ultrasound test.) Government forensic experts pegged Conner's gestational age at between 34 and 40 weeks, and the defense's own experts will almost surely agree. "If this baby was born alive, clearly Scott Peterson doesn't have anything to do with his murder," Geragos told jurors. Prosecutors will present an expert who placed Conner's time of death at about Dec. 25, not January or later. They will say mother and baby separated only after Laci's body decomposed for several months under water. Geragos will counter by maintaining that the tape wrapped around Conner's neck and shoulder could have been put there only by a person tying him up--and therefore happened postpartum. Outside experts--as well as prosecutors--believe that the tide and waves tangled Conner in the water. Says pathologist Dr. Michael Baden, of the New York State Police lab, "Clearly, I think that debris could have been picked up in the water."