The nightmares stirring inside a mother's mind that would drive her to kill her children are beyond comprehension for most people.
But in some cases, psychologists say, extreme depression — possibly brought on after delivering a child — a psychotic breakdown and violence in the home have played a role when mothers have killed their children.
"This is a very rare type of crime," said Jeffrey Smalldon, a forensic psychologist from Columbus, Ohio, when asked to speculate on Andrea Yates' alleged confession that she had killed her five children in the family's Houston home. "There must have been many mental and emotional stress factors operating here."
Experts were careful not to draw any conclusions about the case, and noted that the crime was especially unusual because even when mothers do kill, they almost never kill all their children at one time.
Husband: Wife Had Postpartum Depression
Yates' husband, Russell Yates, told reporters today that his wife was a "kind, gentle person," but suffered from postpartum depression after the births of her last two children.
"What you saw yesterday, that wasn't her," he said, holding a framed photo of his wife and children in front of his chest.
On Wednesday, Andrea Yates, 36, called police to her home in Houston. There, they found the bodies of four boys, aged 2 years to 7 years old, and a 6-month-old girl. All had apparently been drowned, police said.
Mrs. Yates told the officers she had killed her children, police said. Police said she had been taking medication for postpartum depression, but that it remained unclear if her psychological condition may have contributed to the killings.
Russell Yates said his wife was taking two drugs for postpartum depression — efexor and remeron — but still showed symptoms of depression prior to the killings. He said after an earlier pregnancy, she had taken efexor, wellbutrin and haldol for postpartum depression.
Dr. Kim Pearson, a psychiatry instructor at Harvard Medical School and a doctor with the perinatal and reproductive psychiatry unit at Massachusetts General Hospital, said efexor, remeron and wellbutrin are anti-depressants. She added that haldol is an anti-psychotic medication used to treat symptoms such as hallucinations or delusional thinking.
"Certainly, that raises suspicion about whether she had a psychotic incident after the prior pregnancy," Pearson said.
She said 50 percent of women who have postpartum depression have a recurrence in subsequent pregnancies.
"If they've had true postpartum psychosis, than the risk is 75 to 80 percent that they'll have a recurrence," she added. "Certainly, if somebody is experiencing psychotic symptoms, it can lead them to psychotic behavior."
Mr. Yates said his wife had attempted suicide after the earlier pregnancy, but after being treated she had seemed to recover. After the most recent pregnancy, he said, she had seemed to recover to "65 percent" of her normal self and then "she plateaued."
‘A True Mental Break’
On ABCNEWS' Good Morning America today, ABCNEWS medical correspondent Dr. Nancy Snyderman said approximately 10 percent of mothers experience postpartum depression, and psychosis happens in about half of those cases.