Prosecutors opened their case by saying Eric Millerberg, 38, and his wife, Dea, 40, had a drug-fueled, sexual relationship with Alexis Rasmussen that led to the girl's death in 2011.
The North Ogden man has pleaded not guilty to felony child abuse homicide and unlawful sex charges. He faces life in prison.
Dea Millerberg is expected to testify against her husband in 2nd District Court. She filed for divorce in 2012 and awaits trial on charges of desecration of a dead body, according to court records.
Defense lawyer Randall Marshall responded by telling jurors that the evidence against Eric Millerberg comes largely from his client's wife, who he said was lying to protect herself from prosecution. He cautioned the jury to carefully review the evidence.
Dea Millerberg has "use immunity," meaning prosecutors can't use her testimony in this case against her at her own criminal trial in April. She faces five years in prison. She says her husband gave Rasmussen the fatal mix of drugs.
The prosecution, led by Deputy Weber County Attorney Chris Shaw, called several witnesses to testify, led by Rasmussen's mother, Dawn Miera.
Miera said that after her daughter didn't return from babysitting, Eric Millerberg said the teen had gone to meet with a friend at a nearby elementary school, the Salt Lake Tribune reports. Miera later reported her daughter missing, and officials posted the girl's picture around Ogden.
Shaw said Eric "Peanut" Smith came forward about a month after Rasmussen's death, telling police he had helped haul the girl's decomposing body farther off the road in rural Morgan County, home to plummeting ravines and a winding river. He eventually led authorities to the teen's remains.
Eric Millerberg has been in prison awaiting trial. Shaw said that an inmate told police that Millerberg said he and his wife both had engaged in sexual acts with the girl and that he had injected the drugs into Rasmussen's neck, the Tribune reported. Shaw did not identify the informant.
In July, Millerberg was stabbed more than 20 times by his cellmate, the Deseret News reported. That inmate has been charged with attempted murder.
Dea Millerberg has testified previously in evidence hearings that Rasmussen became disoriented after being injected once with heroin and twice with methamphetamine.
She says Rasmussen passed out on a bed and stopped breathing. After Dea Millerberg, a nurse, tried unsuccessfully to revive her, she helped dress the teen and put her body in the trunk of their car.
The Millerbergs put their infant daughter in the car, left their then 6-year-old girl behind and drove off to dispose of the baby sitter's body, she says.
Search warrants allege the couple had long supplied Rasmussen drugs in exchange for baby-sitting.
A jury is scheduled to hear six days of testimony spread out over two weeks.
The Associated Press contributed to this report.