A pregnant woman who had gone to a Chicago home in response to a Facebook offer of free baby clothes was strangled and her baby cut from her womb, US police and family members say.
The newborn is in grave condition and not expected to survive.
Three people were taken into custody, with charges including murder later filed, police spokesman Anthony Guglielmi said.
The body of 19-year-old Marlen Ochoa-Uriostegui was found early on Wednesday behind the house.
The nine months pregnant woman was last seen leaving her high school on April 23, the same day paramedics were called to the home several kilometres away on the Southwest Side about a newborn with problems breathing.
"We believe that she was murdered, and we believe that the baby was forcibly removed following that murder," Guglielmi said, calling it an "unspeakable act of violence".
According to WLS-TV, a 911 dispatcher reported that a 46-year-old woman had called to say that she had given birth 10 minutes earlier and the baby was pale and blue and not breathing.
A mother and her daughter have been charged in the killing.
Clarisa Figueroa, 46, and her daughter, Desiree, 24, were both charged with first-degree murder.
The older Figueroa's boyfriend, Piotr Bobak, 40, was charged with concealment of a homicide.
Police said the younger Figueroa confessed to assisting her mother in strangling Ochoa-Lopez.
The family of Ochoa-Uriostegui, a married mother of a three-year-old son, said a woman on Facebook had lured her to the home by offering a stroller and baby clothes.
"She was giving clothes away, supposedly under the pretense that her daughters had been given clothes and they had all these extra boy clothes," said Cecelia Garcia, a spokeswoman for the family.
Ochoa-Uriostegui's family has been looking for her since her disappearance more than three weeks ago, organising search parties, holding news conferences and pushing police for updates on the investigation.
Her husband, Yiovanni Lopez, visited his son at the hospital and named him Yadiel, the Chicago Sun-Times reported.
"Why did these people, why did these bad people, do this? She did nothing to them," Lopez told WLS. "She was a good person."
Australian Associated Press