"If it's a husband, like you saw in the Scott Peterson case, he sees the unborn child and his wife as an obstacle to the life he wants to lead, a burden, a lifelong obligation of child support, and he doesn't want that lifelong obligation," said Brown. "The only way he sees to solve his problem is to get rid of her. He can't let her live because there is still that lifetime of responsibility."
In the Underwood case, Texas authorities believe Barbee also saw her as an obstacle and a complication. But unlike Peterson, he wanted to stay married and keep his affair and unborn child a secret.
Unlike men, women who attack pregnant women usually do not know their victims well, if at all. They are usually obsessed with pregnancy and crave the attention -- and what they perceive as power -- associated with carrying a child.
Relatives said Lisa Montgomery, of Melvern, Kan., faked pregnancy five times. During the last false pregnancy, she allegedly zeroed in on Bobbi Jo Stinnett, a Missouri woman who was eight months pregnant, strangled her and cut Stinnett's baby from her womb. The child was found alive with Montgomery, who allegedly told relatives she had just given birth. Montgomery now faces a capital murder charge.
"With women who actually want to steal a woman's baby, they are usually psychopaths. They claim to be pregnant when they are not," Brown said. "She usually loves the attention and power that is associated with pregnancy and motherhood. ... They like to use the child to get attention for themselves. But they like to try to manipulate others with the issues that motherhood and pregnancy bring."
Kentucky authorities said Katie Smith told family and friends she was pregnant. She wore maternity outfits and had a completely furnished nursery with baby clothes, diapers and formula.
But there was no pregnancy. To get a baby, police said, Smith, 22, lured neighbor Sarah Brady -- who was nine months pregnant -- to her apartment by telling her a package intended for Brady had been delivered to Smith's home by mistake. When Brady, 26, showed up, Smith tried to stab her, but the pregnant woman managed to turn the knife on her attacker, police said. Smith was killed. Investigators said Brady acted in self-defense, and she was not charged.
Brown said that one myth about women who attack or kill pregnant women is that they are failed mothers or are grieving because of past miscarriages or failed attempted pregnancies.
"This is not a matter of grieving," she said. "They are liars. They've usually had histories of lying about being pregnant. And you have to be a pretty cold-blooded killer to kill a pregnant woman."
To an extent, no matter the sex of the alleged assailant, men and women accused of attacking or killing pregnant women all feel a need for control. However, there is no typical profile and multiple factors can fuel a motive for murder.
"There can be an affair going on, where the husband or boyfriend are getting a lack of sexual gratification and they venture out, fall in love and feel like they have to get rid of the wife," Tod Burke, professor of criminal justice at Radford University in Virginia and a former Maryland police officer, has said in a past report. "But there really is no typical motive in cases like these. It really is situational."