Tiffany Hall, 24, was charged as authorities searched for the dead woman's two sons, ages 7 and 2, and 1-year-old daughter. Authorities say they were last seen with Hall on Monday, three days before she was taken into custody.
Hall faces charges of first-degree murder and intentional homicide of an unborn child in the death of 23-year-old Jimella Tunstall, said St. Clair County State's Attorney Robert Haida.
Hall is being held in lieu of $5 million bail in the St. Clair County Jail in Belleville.
Relatives of both women told media outlets the two grew up together and attended alternative schools. The woman in custody often baby-sat Tunstall's children, and Tunstall never expressed worry about leaving them in her care, the St. Louis Post-Dispatch reported.
An autopsy showed Tunstall bled to death after sustaining an abdominal wound caused by a sharp object, believed to be scissors, said Ace Hart, a deputy St. Clair County coroner. Investigators told CBS affiliate KMOV-TV in St. Louis that the baby was most likely cut from the victim's womb last week.
Hart said he believes Tunstall was knocked unconscious before her baby, seven months into gestation, was removed during a slaying he called "very graphic and very brutal."
Officials were optimistic about the prospects of finding the children unharmed. Illinois State Police Capt. Craig Koehler said authorities will not end the search until the children are found.
"We have no evidence that leads us to believe they're dead," he said, adding two or three additional sites will be searched in and around East St. Louis on Sunday.
Koehler and Mister refused to publicly discuss the evidence or possible statements Hall has made to investigators. Authorities have also refused to reveal how the women knew each other, or whether Hall's alleged confession came before Tunstall's body was found.
Haida did not immediately return messages Saturday seeking information on Hall's arraignment and attorney.
The charges came a day after a meticulous scouring of the 1,100-acre Frank Holten State Park, just blocks from where Tunstall's body was found. Investigators would not say what led them to believe the children were there.
Hall summoned police to the Frank Holten park on Sept. 15, saying she had gone into labor, Hart said. The dead baby, taken to a hospital, showed no signs of trauma, and an autopsy the next day failed to pinpoint a cause of death, he said.
Hall would not let doctors at the hospital examine her and offered conflicting reasons for why she went into labor, alternately saying she had consensual sex and was raped, Hart said.
Authorities say Hall acknowledged to her boyfriend during the baby's funeral Thursday that the child was not his, and that she killed the mother. The boyfriend told police, who arrested his girlfriend hours later, investigators said.
DNA tests should determine definitively whether the baby was the one Tunstall was carrying, Hart said.
The baby was buried Thursday as Taylor Horn after a funeral arranged by L. King Funeral Chapel, whose president said Hall called minutes after the service was to start, asking if she could reschedule for a different day so more relatives could attend. At the time, Levi King said, only two relatives were there.
The woman showed up two hours late, ultimately signing an affidavit for the funeral home stating that the child was hers, King said.