Testifying for the prosecution at a preliminary hearing for Scott Peterson, forensic pathologist Dr. Brian Peterson, no relation to the couple, said Laci Peterson's neck, head, forearms and one of her feet were missing.
He attributed the poor condition of the body to exposure to saltwater and sea creatures.
Dr. Peterson said that her abdomen was worn away, but that there was no evidence she had given birth.
Meanwhile, the judge ruled that prosecutors can use a disputed form of DNA analysis on a hair found in Scott Peterson's boat to try to prove he killed his pregnant wife and dumped her body in San Francisco Bay.
Mitochondrial DNA has rarely been used as evidence in California court cases and it cannot provide a definitive match. Peterson's lawyers challenged the evidence as unreliable, arguing that the statistics used to determine the chances of a genetic match are faulty.
But Superior Court Judge Al Girolami said prosecutors could tell a jury that the mitochondrial DNA from that specific strand of hair could be found in one out of every 112 whites.
The ruling came at the start of the fourth week of the preliminary hearing being held to determine if there is enough evidence against Scott Peterson to try him on murder charges. He could get the death penalty if convicted.
Scott Peterson left the courtroom on Monday when testimony turned to the state of his wife's corpse and the remains of their son.
For Peterson, going to court is the only break he gets from the routine of being in a 6-by-9 foot maximum security cell, and he is a target in jail, reports CBS News Correspondent Manuel Gallegus.
"He's rather notorious and there have been other inmates in our facility who have made mention of wanting to become notorious themselves by either harming him or doing something in relation to this case," Kelly Huston of the Stanislaus County sheriff's department told Gallegus.
Dr. Peterson testified that the fetus was in relatively good condition and had only recently broken free from the mother when the two bodies were found in April. The pathologist said he could find no evidence of wounds.
Scott Peterson's lawyers have argued that Laci, who was eight months pregnant when she disappeared, may have given birth weeks after she vanished.
Dr. Peterson said the prolonged submersion in San Francisco Bay had swollen the body and made his measurement inaccurate. But he testified that an anthropologist who examined the body estimated the fetus to be 33 weeks to 38 weeks old.
Investigators believe a human hair found in pliers in Peterson's boat came from his wife, who disappeared from their Modesto home around Dec. 24 last year. Her body and that of the couple's unborn son washed ashore in San Francisco Bay, about three miles from where Scott Peterson, 31, said he went fishing on the day of Christmas Eve.
FBI forensic experts extracted mitochondrial DNA from the hair and a criminalist with a state crime laboratory testified last week that the hair resembles stands taken from Laci Peterson's brush.
Mitochondrial DNA testing is often used when the biological sample is small or degraded. The technique was used to identify victims of the World Trade Center attack.
Experts say mitochondrial DNA is not as reliable as the more familiar nuclear DNA, which can be used to prove someone's identity. But mitochondrial DNA, if matched with similar samples from a person's mother or sibling, can show a statistical likelihood of identification and rule out others.