However, the expert later acknowledged he could not reproduce the trajectory for Laci's body, only a possible path for the body of the fetus, leaving unanswered the key question he had been asked to determine — exactly where Laci's body had been dumped in the bay.
Prosecutors claim the fetus was expelled after her death while her body lay weighted to the bay floor.
Ralph Cheng, a hydrologist with the U.S. Geological Survey and an expert on bay tides and currents, said that based on winds and tidal information, he predicted the fetus' body — whether still inside Laci or not — had been placed into the bay between Brooks Island and the Berkeley Marina.
However, Cheng added that his theory was "not a deterministic prediction," calling it the "highest probability" location.
"Can you predict with any certainty within inches or feet where these bodies would have started from?" asked prosecutor Dave Harris.
"No, I'm afraid not," Cheng replied.
Defense lawyers later attacked his findings as conjecture.
Prosecutors allege the 31-year-old former fertilizer salesman killed his wife in their Modesto home on or around Dec. 24, 2002, then dumped her weighted body into the bay. Her badly decomposed remains — and that of her fetus — washed up in April 2003, not far from the marina where Peterson launched his boat that Christmas Eve morning for what he said was a solo fishing trip.
Defense lawyers maintain someone else abducted and killed Laci, then framed their client after learning of his widely publicized alibi.
Peterson told authorities he was fishing around the Brooks Island area in the bay.
Prosecutors claim an April storm churned up the waters, dislodging Laci's body from the bay floor. Cheng said his scientific data supports such a theory.
On cross-examination, defense lawyer Mark Geragos quickly attacked Cheng's findings as "assumptions."
"Most scientific information is based on assumptions," Cheng said.
Geragos then noted this was the first such study Cheng had ever performed.
"You've never done any study in San Francisco Bay that has anything to do with bodies or things of that size, correct?" Geragos asked.
"That is correct," said Cheng, who typically studies environmental factors in the bay.
"Basically all of this is largely uncertain, correct?" Geragos asked.
"That is not correct," Cheng replied. "Nothing is 100 percent."
"So it's probable, it's not precise, is that a fair way to characterize it?" Geragos prodded.
"Yes," Cheng replied.
Geragos also noted that one of Cheng's own theories for how the bodies could have washed up apart from each other is that they were placed into the bay separately, supporting the defense theory that the fetus was born alive after Laci disappeared. Such a theory, they say, proves Peterson couldn't be the killer, given Laci's Feb. 10 expected due date.