Prosecutors allege that Scott Peterson killed his wife on or around Dec. 24, 2002, in their Modesto home, then dumped her body into the bay.
Her remains and that of her fetus washed up in April 2003, not far from the marina where he launched his boat on Christmas Eve for what he claims was a solo fishing trip.
Peterson's lawyers maintain someone else abducted and killed Laci.
Dr. Greggory DeVore, an expert in fetal medicine, said he was asked by prosecutors to examine Laci's medical records and bones taken from the dead fetus to help determine when it died.
The age of the fetus is important because prosecutors claim it was expelled dead from Laci's decaying corpse. Defense lawyers maintain the baby was born alive after Laci vanished, proving that her husband could not be the killer given the baby's due date of Feb. 10.
The coroner who performed the autopsy on the fetus estimated its age at death to be about nine months, or full term. A forensic anthropologist testified previously that the fetus' age was between 33 and 38 weeks.
DeVore said the Dec. 23 death date for the fetus was an average taken from several different examinations. The fetus could have also died on Dec. 21 or Dec. 24, he said. He estimated the fetus' age at its time of death to be about 33 weeks and one day.
DeVore never physically examined Laci Peterson before her death.
On cross-examination, defense lawyer Mark Geragos quickly pointed out the "varying results" from different doctors regarding the age of the fetus, and noted DeVore's opinions were simply based on averages and estimates.
"We know that two out of three of these calculations are wrong, is that correct?" Geragos asked, noting that Laci Peterson's baby was still alive when she visited her doctor on Dec. 23.
DeVore agreed but defended his estimates.
Geragos also noted to the obviously flustered DeVore that a test using another scientific method would indicate the fetus died on Dec. 28.
On Tuesday, Modesto police Detective Craig Grogan said police had more than enough reason to suspect Scott Peterson was involved in his pregnant wife's disappearance, and that her body lay somewhere in San Francisco Bay.
Grogan ticked off the list of reasons in rapid succession, providing jurors with the first detailed narrative of the murder case against Peterson in the 17-week trial. Grogan's account had jurors leaning forward in their seats, scribbling notes.
He explained each point:
Grogan then looked toward the jury as he completed the list.
"The ultimate conclusion was that Laci Peterson's body was in San Francisco Bay and that we needed to search there," Grogan said.
Despite all these clues, Grogan noted that police remained open to other possibilities. "We had a policy that if someone called in and said they knew where Laci Peterson's body was ... that we would go there and search it."
Grogan was due back on the stand later Wednesday.