A pregnant teenager who went 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 said.

The newborn was in a grave condition and not expected to survive while murder charges have been filed against a woman and her daughter.

Police said 46-year-old Clarisa Figueroa and her daughter, Desiree Figueroa, 24, have been charged with first-degree murder.

The older woman’s boyfriend, 40-year-old Piotr Bobak, is charged with concealment of a homicide.

The body of 19-year-old Marlen Ochoa-Lopez was found early on Wednesday behind the house, more than three weeks after she disappeared.

The teenager, who was nine months pregnant, was last seen leaving her high school on April 23, the same day paramedics were called to the home several miles away on the Southwest Side about a newborn with problems breathing.

CORRECTION Pregnant Woman Slain Baby Taken
Chicago Police put out a missing person flier for Marlen Ochoa-Lopez before she was found dead (Chicago Police/Chicago Tribune/AP)

“We believe that she was murdered, and we believe that the baby was forcibly removed following that murder,” a police spokesman said, calling it an “unspeakable act of violence”.

Chicago Fire Department spokesman Larry Merritt said a 46-year-old woman called 911 reporting that her newborn baby was in distress.

When paramedics arrived “the baby wasn’t breathing, the baby was blue,” said Mr Merritt. Paramedics tried to resuscitate the baby on the way to the hospital.

The family of Ms Ochoa-Lopez, 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.

Pregnant Woman Slain-Baby Taken
Marlen Ochoa-Lopez’s husband, Yiovanni Lopez, talks to reporters outside the Cook County medical examiner’s office after identifying his wife’s body (Ashlee Rezin/Chicago Sun-Times/AP)

“She was giving clothes away, supposedly under the pretence that her daughters had been given clothes and they had all these extra boy clothes,” said Cecelia Garcia, a spokeswoman for the family.

Ms Ochoa-Lopez’s family had been looking for her since her disappearance, organising search parties, holding news conferences and pushing police for updates on the investigation.

A break in the investigation came after the woman who said she had given birth to the baby set up an online fundraising campaign, another spokeswoman for Ms Ochoa-Lopez’s family said.

The GoFundMe campaign said that the baby was about to die and money was needed for a funeral, Sara Walker said.

Police then conducted DNA tests to determine that Ms Ochoa-Lopez and her husband, Yiovanni Lopez, were actually his parents, Ms Walker said.

Mr Lopez has been visiting his son at the hospital.

“Why did these people, why did these bad people, do this? She did nothing to them,” Mr Lopez told reporters in Spanish on Wednesday night. “She was a good person.”