A US woman used a stun gun to subdue a pregnant woman before killing her and cutting the baby boy from her body, a police investigator has testified.

Kentucky State Police Detective Chad Winn, speaking during a preliminary hearing in Warren County District Court today, said Kathy Michelle Coy told 21-year old Jamie Stice they were going shopping for baby supplies but took her instead to a wooded area with plans to kill her.

Ms Coy, 33, has been charged with murder and kidnapping of a minor.

Detective Winn testified that after attacking Ms Stice with the stun gun, Ms Coy slit Ms Stice's throat and wrists, then cut the baby out of her abdomen.

Detective Winn said Ms Coy eventually led investigators to Ms Stice's body and gave details of the slaying.

"She advised that she stunned her, her throat was slit, her wrists were slit, she was disemboweled," Detective Winn said as members of Ms Stice's family fled the courtroom in tears. "The baby was obviously cut out of her."

Ms Stice's remains were found off a dirt road in southern Kentucky on April 14, less than a day after she was last seen leaving her home with Ms Coy.

Ms Coy was arrested after showing up at a Bowling Green hospital with a newborn baby, but no signs of having given birth, Detective Winn said.

Warren County District Judge Sam Potter sent the case to the grand jury. Ms Coy's public defender, Jim Gibson, objected to the preliminary hearing, saying Ms Coy's competency and mental state were uncertain.

"The defence is concerned that there are significant mental health issues," Mr Gibson said.

Warren County Commonwealth's Attorney Chris Cohron said he may seek the death penalty for Ms Coy, but will make that decision after any mental health issues are litigated. Mr Cohron said the case will be presented to a grand jury within two months.

James Reynolds, the baby's father, says the infant is in good condition and that his name is Isaiah. He declined further comment.

Detective Winn was the only witness to testify during the 30-minute hearing.

Ms Coy, who has a hearing impairment, did not appear to look at or acknowledge Ms Stice's family, but did cry before the hearing.

Detective Winn said there is evidence that Ms Coy, who claimed to have had a miscarriage, planned to kill a pregnant woman and steal a baby.

Detective Winn said Ms Coy approached her 13-year-old daughter in early April about taking part in such a plot.

When the daughter baulked, Ms Coy passed it off as a joke, Detective Winn said.

Soon after, Ms Coy approached her 14-year-old son and asked if he would take part in a murder, Detective Winn said. The son refused and Ms Coy didn't bring it up again, the detective testified.

After killing Ms Stice and stealing the baby boy, Ms Coy drove to the home of a friend and said she had given birth to the baby, Detective Winn said.

The friend told troopers that Ms Coy was in a car, wearing no pants and sitting on the placenta while holding the baby, who had grass on him, Detective Winn said.

The friend called for an ambulance after helping to clean the baby and taking a picture of the newborn, Detective Winn said.

At the hospital, at about 5.50pm local time on April 13, Ms Coy brought in a uterus, ovaries and placenta with umbilical cord still attached, along with the baby, Detective Winn said.

At the hospital, Dr Sara Mangold examined Ms Coy and determined she didn't give birth to the baby, Detective Winn said.

"She showed no signs whatsoever of having a child," Detective Winn said.

After seeing the internal organs Ms Coy presented to doctors, hospital staff suspected someone had been killed and called police, Detective Winn said.

Ms Coy initially insisted she gave birth to the boy, then told police she bought the baby for $US550 from a woman named "Ashley", but provided few details, Detective Winn said.

Ms Coy eventually led detectives to a wooded area off a dirt road, where Ms Stice's remains were located, Detective Winn said.

Originally published as Baby cut out of mother