The mother of the dental hygienist who led police on a high-speed chase near the Capitol before she was shot and killed on Thursday told ABC News that the 34-year-old Miriam Carey suffered from postpartum depression.

But it's unlikely that postpartum depression could account for such bizarre behavior, experts said. A separate and rarer condition, however, called postpartum psychosis, can manifest in extreme -- and sometimes violent -- manners.

"It's the loss of touch with reality," said psychotherapist Diana Barnes, an expert on perinatal mood disorders. "Women suffering from it generally hear voices. Sometimes the voices are commanding, they dictate behavior, often very critical and demeaning. The mother comes to believe that she is in harm’s way or the baby is."

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While the term postpartum depression is often used to encompass an array of emotional effects a mom experiences after giving birth, ranging from mild baby blues to incapacitating depression, affecting as many as 15 percent of new mothers, postpartum psychosis affects fewer than 1 percent. But of those, 5 percent commit suicide, and 4 percent commit infanticide.

"Postpartum psychosis is a mental health emergency,” said Suzanne Swanson, a licensed psychologist in Minnesota and a local coordinator for Postpartum Support International. "There's a real break from reality that can prompt bizarre behavior; a woman may think 'someone's out to get me.'"

The disorder can come on suddenly, and is often misdiagnosed or overlooked.

"At one moment she can look very lucid, and at the next moment, her thoughts start to unravel," Barnes said.

Post-partum depression can include anxiety, panic, obsessive compulsion, even post-traumatic stress disorder, Swanson said. Although psychiatric episodes are more common in the childbearing years than any other time in a woman's life, Barnes said, postpartum psychosis is often linked to mental illnesses from a woman's past. Pre-existing history of disorders such as bipolar disorder or schizophrenia increase a woman’s vulnerability to postpartum psychosis, Barnes said.

It's impossible to know what prompted Miriam Carey's actions.

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"Her mother could be saying postpartum depression and meaning psychosis," Swanson said.

Another distinguishing feature: Mothers with postpartum depression may have repetitive thoughts about harm coming to the baby, but the moms recognize that the thoughts are bizarre -- and then go to great lengths to avoid the situations coming to fruition. For example, a mother who imagines dropping her baby may stop holding her, Barnes said.

"Just because you have an illness doesn't mean it's going to end in tragedy," Barnes said. "Not every woman is going to demonstrate bizarre behavior or kill her baby. We have to really look at this case by case and really understand it's a very big problem; the surface is just being scratched here in terms of this woman's history."

Swanson encourages anyone who may be experiencing symptoms or knows someone who needs help to get in touch with Postpartum Support International (1-800-944-4PPD).