In an interview with ABC News, the father of missing Oregon 7-year-old Kyron Horman suggests that the boy's stepmother may have suffered from some form of post-partum depression.
Kaine Horman called Terri Horman's emotional state "more erratic" after giving birth to their daughter 19 months ago, and wonders if that may have played a role in young Kyron's disappearance.
Doctors say for a few women, what begins as mild depression can escalate over time to psychosis, impairing judgment, and in rare and tragic cases, it can lead them to kill, ABC News reported.
An example is Andrea Yates, who had a history of postpartum depression when she drowned her five children in her Texas apartment in 2001.
A postpartum psychosis case in Georgia last month ended in a 15-year prison sentence for Joanne Tucker, who beat her infant daughter to death.
While there is no confirmation of their claim, Kyron's parents said they believe postpartum depression could be a factor linking the stepmother Terri Horman to Kyron's disappearance, ABC News reported.
"I just really want her to do the right thing," said Kyron's biological mother, Desiree Young. "I can't say it enough, that Kyron is still out there and he needs to come home."
Investigators have not named Terri Horman as a suspect, or charged her with any crime. Both she and her attorney declined to comment for this report.
Louann Brizendine, a neuropsychiatrist and author of "The Female Brain," told "Good Morning America" that for as many as one out of 20 women, the huge change in hormones after giving birth can lead to postpartum depression.
"Another group, one out of 1,000, may develop more severe mental illness called postpartum psychosis, and that is the one that's more associated with women killing their children and fantasizing (about killing their children)," she said, using the Andrea Yates case as an example.
Postpartum depression symptoms are marked by a lot of time spent crying, irritablity, depression or having no energy to care for self or baby, Brizendine said.
"Postpartum psychosis is where the mother actually loses touch with reality, bizarre thinking, strange ideas and may have what seems to be a complete personality change," she said. In the context of losing reality, a mother may think that "somehow the baby is evil and the baby may need to be killed to save it from going to hell."