Laci Peterson's fetus likely died around Dec. 23, 2002, a day before the pregnant schoolteacher was reported missing, an expert testified Wednesday, noting the date was an average estimate.

Dr. Greggory DeVore (search), an expert in fetal medicine, said he was asked by prosecutors to examine Laci Peterson's (search) 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 their client, her husband, Scott Peterson (search), couldn't 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 she estimated the fetus' age to be 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 (search) 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 "about 41 reasons" in rapid succession, providing jurors with the first detailed narrative of the double-murder case against Peterson in the 17-week trial.

Prosecutors allege Peterson killed his eight-months pregnant wife on or around Dec. 24, 2002, in their Modesto home, then dumped her weighted body into the bay. Her remains — and that of her fetus — washed up in April 2003, not far from the Berkeley Marina where Peterson launched his boat on Christmas Eve for what he claims was a solo fishing trip.

Defense lawyers maintain someone else abducted and killed Laci.

But Grogan's account had jurors leaning forward in their seats, scribbling notes.

He explained each point:

— "The dog tracking at the Berkeley Marina that indicated Laci Peterson's scent was there."

— "The defendant told us that he was at the Berkeley Marina."

— "He had a two-day fishing license that was purchased on Dec. 20 and filled out for the 23rd and 24th."

— "The fishing tackle in the boat ... was freshwater tackle."

— Peterson told some witnesses on the night Laci vanished he had been golfing all day. "We considered that possibly was what his initial alibi was meant to be."

— Peterson loaded large umbrellas into the back of his pickup truck that Christmas Eve morning. "It would enable him to be able to explain to anyone seeing him load something in his truck."

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.

Grogan also detailed a Feb. 18 search of the Petersons' home when police discovered Peterson had packed bags. Stuffed in one pouch was his wedding ring, Grogan said. Police also found $2,081 in cash in the bags.

Grogan said detectives were looking for, among other things, "potential poisons."

On one wiretapped phone call played for jurors, Peterson apologizes to Grogan for not being honest about his affair with massage therapist Amber Frey. Peterson didn't come clean with police about the affair — his motive, prosecutors allege, for the murder — until after he was confronted.

Then came a "Good Morning America" interview on Jan. 28 where Peterson is again caught in a lie when he tells ABC's Diane Sawyer that he told police about his affair on the first night Laci was reported missing.

Jurors had just heard Grogan tell them that was a lie.

A day later, Grogan called Peterson to confront him about the interview. In their recorded conversation, Peterson immediately acknowledges the lie before Grogan brings it up.

"They caught me answering a question about ... that I told you about a girlfriend ... is not true. We both know that," Peterson tells the detective, as if he had just misspoken and was simply caught off-guard in the interview.

On the taped call, Peterson begins to sob uncontrollably.

"I'm losing it. I miss her ... I'm just a mess without her," Peterson tells Grogan.

The detective then attempts to elicit a confession from Peterson, saying "You and I both know what happened to Laci."

"Do you know what happened to her?" Peterson replies.

"We both do," Grogan says sternly, but in a soft-spoken voice.

"I want the door open between us," Grogan continues. "If you want to end all of this nonsense, all you need to do is call me, all right? We can sit down. I will not treat you badly. You can tell me what happened. We can get Laci back where she needs to be."

"I'm not involved in my wife's disappearance," Peterson insists, his sobs now replaced with a firm, steady voice. "We're going to find her."