The suspect allegedly put Marlen Ochoa-Lopez's body in a large plastic bag and placed it in a garbage can in her backyard, police said.
Suspects Clarisa Figueroa, 46, and her daughter, Desiree Figueroa, 24, were charged with first-degree murder and aggravated battery in the attack that happened on April 23, Chicago police said.
The mother and daughter were arrested this week after investigators found the teen's body in a trash can at the elder Figueroa's home, police said.
After they allegedly killed her, Clarisa Figueroa pretended the baby was hers and intended on raising him as her own after her son died last year, authorities said.
The baby boy is hospitalized in grave condition and is on life support, Chicago police said Friday.
In addition to the mother and daughter, Clarisa Figueroa's boyfriend, Piotr Bobak, 40, was charged with concealing the death of a person and concealing a homicidal death.
All three are being held without bond.
The suspects allegedly planned the attack for months.
Clarisa Figueroa's 20-year-old son Xander died in 2018, according to prosecutors. In October, she announced she was pregnant, which came as a surprise to her family because her fallopian tubes were tied.
By December, Clarisa Figueroa had posted a Facebook picture of an ultrasound showing what she claimed was the baby she was carrying, assistant state's attorney James Murphy said.
Months later, in February, she posted a picture of a crib and baby's room to a Facebook page that provides baby items for families in need, Murphy said.
The next month, she posted on the page again, asking, "Who is due in May" and "Where is the May mama's at."
One of the people she connected with was Ochoa-Lopez, who at that time was seven months pregnant, Murphy said. The teen had posted that she was looking for baby items, including a stroller. Clarisa Figueroa offered her clothes for the baby and suggested they talk in a private message.
Then on April 1, Clarisa Figueroa told her daughter that "she needed help killing a pregnant woman and taking a baby," Murphy said.
The high school student went to Figueroa's home at least twice this year after connecting on the Facebook group and accepting Figueroa's offer of baby items, police said.
One of those visits -- the second and last one -- was on April 23, Murphy said.
As Ochoa-Lopez sat on a couch and Desiree Figueroa distracted her with a photo album, her mother wrapped a cable around the teen's neck from behind, Murphy said.
When Ochoa-Lopez put her fingers between her neck and the cable, the elder Figueroa told her daughter that she wasn't doing her job, Murphy said.
Clarisa Figueroa then got on top of the teen and strangled her for about four to five minutes, the prosecutor said.
When she was dead, the elder Figueroa cut the teen's baby from her womb. She then called 911 and said she'd just delivered a baby that wasn't breathing, according to Murphy.
Both were hospitalized, but Clarisa Figueroa showed no signs that might have suggested she had just delivered a baby, Murphy said.
Ochoa-Lopez's husband reported her missing on April 24, police said.
But it would take nearly three weeks for investigators to unravel what happened and find Ochoa-Lopez's remains in a trash can in the backyard of Clarisa Figueroa's home, authorities said.
Police said a break in the case came May 7 when a friend of the victim told detectives about Ochoa-Lopez's exchanges with Figueroa before she vanished.
Detectives visited the Figueroa home that day, and Desiree Figueroa told them her mother was in the hospital for problems with her legs. Then she revealed that her mother had just delivered a baby, authorities said.
The investigators searched the area and found Ochoa-Lopez's vehicle nearby, Deputy Chief of Detectives Brendan Deenihan said.
That same day, detectives interviewed the elder Figueroa at a hospital. She denied the victim came to her house April 23 but admitted to meeting her in the past, police said.
Suspicious, detectives spent the next several days subpoenaing hospital records and collecting DNA samples from the baby and Figueroa.
DNA tests determined Clarisa Figueroa was not the mother of the baby and Ochoa-Lopez's husband, Yovani Lopez, was the father, Deenihan said.
The victim's family said they notified authorities about her interaction with the woman much sooner than May 7.
Her husband tried to immediately report her missing to the police, but was told to come back in 72 hours, according to Jacobita Cortes, a pastor of a Chicago church that the family had asked for help.
The husband did, and the family hired a private investigator who found Ochoa-Lopez's car near the Figueroa home, Cortes said.
"There's going to be anger associated," Chicago police Superintendent Eddie Johnson said. "When things of this nature occur, the first thing people do is look in retrospect, what could we have done to maybe prevent this? I know our detectives do the best they can."
Armed with a search warrant, crime lab officers searched Figueroa's house Tuesday.
Detectives discovered bleach and cleaning solutions in the home, along with evidence of burned clothing and blood on the floors of the living room, bathroom and a hallway, Deenihan said.
Ochoa-Lopez's body was found in a garbage can in the back yard, along with a cable used to strangle her and other evidence, police said.
A medical examiner determined that she died of strangulation, and Desiree Figueroa told detectives she helped her mother kill her, police said.
The motive for the killing is under investigation, but Johnson said he believes the elder Figueroa wanted to raise the child as her own.
CNN's Dakin Andone, Ray Sanchez and Andrea Diaz contributed to this report.