Pregnant Rhoni Reuter was killed in a coldly calculated plot by a jealous love rival, desperate for the affection of Shaun Gayle, the former Chicago Bears player from the 1985 Super Bowl team.
"Evidence will show the defendant became obsessed with Shaun Gayle," said Patricia Fix, assistant state's attorney, told the rapt jury.
"She devised a plan to eliminate her perceived competition."
She added that Yang had been a part time lover of Gayle, who now works as a part time real estate agent.
Reuter's pregnancy "was a game changer," she said, so back in October 2007, Yang went round to her Deerfield condominium and blasted her with a pistol, aiming the first two shots at Reuter's abdomen and killing the unborn baby girl.
Fix told jurors that Yang had carefully planned the murder and how to cover her tracks, ordering a book two months before the shooting on how to build a silencer for her 9 mm Beretta.
She was six months pregnant at the time.
They claim Yang disguised herself by darkening her complexion, wearing baggy clothes and a wig before driving to her condo in a rented car that had stolen license plates.
Then with the help of her friend, Tarot reader, Christi Paschen, she got rid of the evidence at several places.
At the last stop they buried one of Reuters bracelets, Fix said - a place where Paschen led cops, which ultimately led to Yang's arrest.
Paschen was later convinced by Cops to wear a wire and in March 2009, police eavesdropped as Yang revealed details about the murder to Paschen while the two sat in a Denny's restaurant.
Fix told the jurors that they would hear some of the taped conversations between Yang and Paschen.
She said: "You will hear the defendant's own words resonate in the room."
Defense lawyer's however, painted a different picture of Yang, a divorced mother of three.
Describing Gayle as a womanizer with 18 to 19 girlfriends he claimed Cops had ignored other suspects and focused exclusively on her.
William Hedrick said: "Marni Yang was not obsessed with Shaun Gayle. She understood Shaun Gayle had many girlfriends.
"The real obsession in this case is the obsession of the Lake County Crimes Task Force with Marni Yang as a suspect."
He also questioned the credibility of Tarot reader Christi Paschen, who alleged that her close friend had confessed to the killing and taped the conversations.
The tapes, he said, reveal only "a festival of liars" he said.
The Trial continues.