A pregnant woman who responded to an offer of free baby clothes on Facebook was strangled and had her child cut from her womb, say Chicago police.

The body of Marlen Ochoa-Lopez, 19, was found behind a house in Chicago on Wednesday, three weeks after she was last seen leaving her high school.

"We believe that she was murdered, and we believe that the baby was forcibly removed following that murder," said police spokesman Anthony Guglielmi.

The baby is said to be in a serious condition and is not expected to survive.

Three people are in custody. Two, 46-year-old Clarisa Figueroa and her 24-year-old daughter Desiree, were charged with murder. The mother's boyfriend, 40-year-old Piotr Bobak, was charged with concealment of a homicide.


Image: The 19-year-old went missing three weeks ago and was last seen leaving school

The victim's family said she had been lured to the home by a woman on Facebook who was offering baby clothes and a pushchair.

"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 family spokeswoman Cecelia Garcia.

Ms Ochoa-Lopez's family had been organising search parties and publicising her disappearance.

The breakthrough came when a woman set up a fundraising page saying she needed money for a funeral because her baby was about to die, according to Sara Walker - another spokeswoman for the victim's family.

Image: Ms Ochoa-Lopez's husband has visited his son in hospital

The same woman had called emergency services the day Ms Ochoa-Lopez disappeared to say her newborn was having trouble breathing.

The baby was taken to hospital and DNA tests later found the baby actually belonged to Ms Ochoa-Lopez.

Yiovanni Lopez, the baby's father, has been visiting his son in hospital.

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

Ms Ochoa-Lopez was already mother to one young son.