Kansas City, Mo. ? A mental health expert insisted Friday that Lisa Montgomery’s confession is “proof positive” that she was not criminally insane when she killed a pregnant woman and cut the baby from her womb.

Dr. Park Dietz, the final witness to testify, discounted defense claims that Montgomery was suffering from pseudocyesis, which causes a woman to falsely believe she is pregnant and exhibit outward signs of pregnancy.

Defense mental health experts have said threats to Montgomery’s delusion about being pregnant caused her to enter a dreamlike, dissociative state when she strangled Bobbie Jo Stinnett on Dec. 16, 2004.

But federal prosecutors say Montgomery is faking mental illness to escape a possible death sentence, which they plan to pursue if she is found guilty of kidnapping resulting in death. Jurors will begin deliberating the case after the attorneys make their closing arguments Monday.

Dietz, a rebuttal witness for the prosecution who began testifying Thursday, said Montgomery knew she wasn’t pregnant and felt increasing pressure to produce a baby as her purported due date approached.

He noted that Montgomery’s ex-husband, Carl Boman, had told her he suspected she was faking the pregnancy and planned to use it against her to obtain custody of two of the couple’s four children. A custody hearing had been set for January 2005.

“I believe the confession is proof positive that my theory is the correct one,” said Dietz, who has worked on other high-profile cases, including those of child killers Andrea Yates and Susan Smith, serial killer Jeffrey Dahmer and Unabomber Ted Kaczynski.

Montgomery, 39, of Melvern, Kan., confessed shortly after her arrest the day after the killing.

On Thursday, Dietz recalled that Montgomery told investigators she brought a knife, rope and umbilical cord clamp with her to Stinnett’s home in the northwest Missouri town of Skidmore. She also took off her wedding ring and left it in the cup holder before entering the home, suggesting she did not want to contaminate it, Dietz said.

Under cross-examination Friday, defense attorney Fred Duchardt asked Dietz whether he had made up his mind about Montgomery’s sanity before interviewing her.

Duchardt pointed out an appearance Dietz made on CNN’s “Anderson Cooper 360” shortly after Stinnett’s death. During the show, Cooper described a study that examined similar killings of expectant mothers and asked Dietz to comment about the type of woman who would commit such a crime.

“They’d be manipulating and taking advantage of people in a variety of ways their whole lives,” Dietz said. He said the women shared “an instinctual drive for motherhood” but “go about it in the most maladaptive way possible.” He also said the crimes were not the result of mental illness.

The prosecution hired Dietz the next week to examine Montgomery.