The poor condition of many bodies recovered from water can make it hard not only to identify them but to tell whether they were victims of violence, forensic experts say.
Also, a phenomenon sometimes called coffin birth can explain how a fetus becomes separated from its mother after death.
A baby -- authorities called it a "full-term male child" -- was discovered Sunday along the eastern shore of San Francisco Bay, near Richmond, and a woman's body was found the next day about a mile away.
DNA testing will determine if the baby was that woman's child.
"One should be careful speculating about some horrible murder when it sounds like natural expulsion," said Dr. Michael Baden, co-director of the New York State Police Medicolegal Investigation Unit and a former New York City medical examiner. In the past, he assisted O.J. Simpson's defense team and Chandra Levy's family, after the Modesto woman's remains were found in Washington, D.C., almost a year after she disappeared there.
Natural expulsion can occur several weeks or months after the death of an unembalmed pregnant woman, he said, when the gases of decomposition push the fetus from the womb.
In the case of the woman's body found near Richmond, authorities are not saying whether it had a head. But Baden said a headless torso would not necessarily point to crime.
That is simply because bodies often do not remain intact, especially in the ocean.
"One of the first things to fall off in decomposition is the head," said Dr. Greg-ory Schmunk, Santa Clara County coroner. He presided over the DNA identification of 7-year-old Xiana Fairchild, whose skull was found two years after she disappeared in 1999. That recovery did not involve water.
Waves pushing bodies on rocks and sand cause further degradation, Schmunk said. More damage from crabs, fish and other foragers is common as well.
"As far as the ocean is concerned, it's food," said Chris Patton of Stanford University's Hopkins Marine Station.
Fingertips fade within a week or two, Schmunk said, making identification from prints impossible.
Absent fingerprints and comparing teeth with dental records, the most common means of identification is X-raying a body, Baden said. The X-rays can easily be compared to, say, a known chest X-ray of a missing person, he said.
After that, "DNA is probably what it's going to take," said Dean Wideman, chief science officer with NucleoGenix, a private consulting firm in San Antonio. He and Baden said bodies retrieved from saltwater usually produce usable DNA samples that can be compared to known samples from a suspected victim's hairbrush or toothbrush, for example.
"Techniques are very good now at typing compromised DNA," agreed Norah Rudin of Berkeley-based Forensic DNA Consulting.
DNA analysis takes three days at a minimum and often much longer, Baden and Schmunk said.
"Fortunately for the family's sake, people are going to be throwing a lot of resources to solving this because it's a high-profile case," Schmunk said of the Peterson investigation.
Baden said April is a common month to recover bodies that entered water during winter months. That is because bacteria begin to thrive as water temperatures rise to 40 degrees, producing decomposition gases that float bodies to the surface.
Schmunk said even homicide victims weighted down can surface eventually. Their feet, for example, can naturally separate from legs, or legs from torsos.
"A lot depends on whether she was submerged or floating," Schmunk said of the body recovered Monday. "My guess is she was not floating very long or she would have been seen. It's much more likely she was submerged for a time, then washed to the surface."
Although limbs naturally separate in time, it is extremely rare for all to disappear unless the person fell victim to a crime, Baden said.
He said authorities should be able to tell immediately if a head was severed as opposed to separation resulting from decomposition or tidal activity.