While it is rare for a mother to kill her newborn, experts say 15 percent of all women feel depressed after pregnancy.

Many of us find it difficult, maybe impossible, to understand how any illness could cause such a horrific act - a mother, killing all five of her children. But "NBC Dateline" spoke to some women who say, while they would never do what Andrea Yates did, they do know some of what she was feeling. They understand, because they too have been there.

"AS I WAS changing my son on his changing table, an intrusive thought started running through my head, What if I push him off the table?" says Randy.

Stacey says, "If I just got up and walked out of the house and never came back again, I really believe that my daughter and my husband would be so much better off."

Esther says, "I can't live this way, this is hell. And I need to get some help."

Karen says, "If a stranger comes in and abducts this child, am I going to cry? Because that means I love this child."

These women know something about Andrea Yates that many of us do not. Their illnesses weren't as severe as hers, but they say they understand the nightmare of postpartum depression.

They looked like a big happy family with four boys and a baby girl. But according to her medical records, Andrea Yates had a history of postpartum depression and psychosis. She had been hospitalized four times, most recently, last year in March and May following the birth of her youngest child, Mary.

On June 20, 2001 in a period of an hour, Andrea Yates drowned her five children in the bathtub of her home in a Houston suburb. She then called the police and her husband.

I said 'Is anyone hurt?' and she said 'Yes,' and I said 'Who?' and she said 'The children,- her husband, Rusty, said.

Many were shocked that a mother could commit such an unspeakable and inconceivable act. Yates' lawyer says she was suffering from a very severe form of postpartum psychosis at the time of the killings.

While post partum psychosis is rare and extremely dangerous, postpartum depression - or PPD - is more common. In fact, experts say it affects up to 15 percent of new mothers.

Cause of Postpartum Depression Still Unknown

There is no singular known cause of post-partum depression or psychosis. Some doctors think hormones might play a role, and experts agree that biological and social factors contribute to the illness. The biggest factor may be whether the mother or her family has a history of mental illness. The birth of a child could possibly act as a trigger for an underlying, pre-existing condition. Treatment involves drugs and therapy, and in severe cases, hospitalization.

These women say the depression they experienced was devastating, debilitating and very frightening.

"It still scares me that I got to that point," says Karen.

As her family grew, Karen fell apart, and didn't know why. Nine weeks after the birth of one of her children, maternal bliss dissolved into feelings of anger, guilt and depression.

Did Karen have any thoughts of hurting her children at any point? "One of my children, I did,- she says. "I would look at the baby and just say, oh, how vulnerable it is. I could put a pillow over the top of it. Its neck was so tiny, it could break so easily."

She and her husband always wanted children so she couldn't understand why she had those thoughts or where they came from and she was embarrassed to even admit she had them.

continue: Signs of Postpartum Depression and Other Mothers Talk About How This Could Happen to Any Woman