Defense attorneys say Lisa Montgomery is a mentally ill woman who sincerely believed she was pregnant when she killed an expectant mother and cut the baby from her womb.

Prosecutors call her a liar who spent months researching Caesarean sections on the Internet before attacking Bobbie Jo Stinnett on Dec. 16, 2004, in the northwest Missouri town of Skidmore. They say Stinnett was conscious and trying to defend herself as Montgomery used a kitchen knife to crudely cut the baby from her womb.

Both sides will make their cases to jurors during closing arguments Monday.

Montgomery, 39, has pleaded not guilty to kidnapping resulting in death. Her attorneys are presenting an insanity defense, claiming she was suffering from pseudocyesis, which causes a woman to falsely believe she is pregnant and to exhibit outward signs of pregnancy.

Mental health experts testifying for the defense say threats to Montgomery's delusion about being pregnant caused her to enter a dreamlike, dissociative state when the murder took place.

Montgomery had undergone a tubal ligation in 1990 after the birth of her fourth child. But soon after, she began claiming to be pregnant again.

The defense said some threats to Montgomery's delusion came from her ex-husband, Carl Boman, who had become suspicious of her latest pregnancy claim and threatened to use it against her as he sought custody of two of the couple's four children. A custody hearing had been set for January 2005.

Montgomery's mother and sister also had been telling her husband, Kevin Montgomery, and his parents that it was impossible for Montgomery to carry a child.

While the defense sees those things as threats to Montgomery's delusion, the prosecution sees a motive for the killing.

As Montgomery's purported Dec. 13, 2004, due date approached, she began conducting searches on the Internet about Stinnett and researching different aspects of child birth. The defense presents those efforts as evidence that she believed she was pregnant. The prosecution offers them as proof of premeditation.

Montgomery was arrested the day after the killing. She had spent the morning showing off Stinnett's baby as her own.

After initially denying the crime, she told investigators she had taken a knife, rope and umbilical cord clamp with her to Stinnett's home. She also said she took off her wedding ring and left it in the cup holder in her car before entering the home.

Montgomery said she thought she was leaving Stinnett's home, but "something out of character" happened and "then this took place." She also told investigators that as she drove away from the Stinnett home, she "could not breathe" and was "shaking and holding the baby and the cord at the same time."

Prosecutors said they will seek the death penalty if Montgomery is convicted.

Besides convicting or acquitting her, jurors could find Montgomery not guilty by reason of insanity. If that is the verdict, she would undergo a mental evaluation and a judge would decide if she will be released or committed to a mental institution.