Ali Bergstrom heard "code blue" blare over the hospital loudspeaker, but it wasn't until the nurses came swarming in that she realized it was her 4-month-old son they were talking about. As the doctors struggled to revive the infant, Bergstrom stood helpless in the corner, saying over and over "Mommy loves you."

That was two years ago. Bergstrom's son, Chille, had heart surgery, recovered, and today is a thriving toddler who is learning Arabic and Mandarin. But in the months immediately following his brush with death, it was Bergstrom who couldn't get back to normal.

She couldn't sleep, shaken awake by nightmares that they were back in the intensive care unit. She couldn't watch her son sleep at night because to her, he looked dead.

"People would be going on with their lives all around me and none of it made any sense to me," the Long Island, NY, mom remembers. "It was like they were speaking a different language or living in a different world. I was completely numb."

Bergstrom was eventually diagnosed with post-traumatic stress disorder, the result of her son's trauma and the months she spent nonstop in the intensive care unit. She takes medication to help her sleep and says therapy has taught her to live in the moment and rely on others.

But most importantly, she says, she learned that being there for her son meant taking care of herself.

"I had focused so much on his needs that I completely had ignored my own," says Bergstrom, who started a blog and is currently writing a book about her experiences. "I thought that was what being a good mom was. It's not."

Bergstrom isn't alone. In a study released last week from researchers at the Children's Hospital of Philadelphia, more than one-third of parents suffered from PTSD after a traumatic injury to their child. Parents reported reliving the event, avoiding places or situations that reminded them of it, and overall feelings of jumpiness and anxiety.

Parents who actually saw their child in pain or thought their child's life was in danger were far more likely to experience symptoms.

That's not surprising, says Dr. Cassandra Kisiel, a psychologist at Northwestern University's Feinberg School of Medicine and a trauma consultant with the state's Department of Children and Family Services.

"Parents understand that their primary job is to keep their kids safe," she says. "Even just a fever or a stomach ache can send anxiety levels spiking, so imagine what a serious injury or a life-threatening illness can do to a parent emotionally."

And it's not just single traumatic events that can trigger post-traumatic stress disorder, says Dr. Astrid Herard, a pediatric psychiatrist at the University of Chicago. For parents of children with chronic diseases, the years spent cycling in and out of hospitals and doctors offices can take a collective toll and produce similar symptoms.

"There's the guilt at not being able to help, and then the trauma itself of years of invasive procedures or extended hospitalizations," she says. "There's all this focus on the child that it can build and build on the parents."

Symptoms that interfere with daily functioning or last more than a month after the incident are cause for concern, both doctors say. PTSD, which is most often diagnosed in war veterans or survivors of violent traumas, can be treated with drugs and therapy.

"It can feel like your whole world is crashing down," Bergstrom says. "But once I understood and accepted what was happening to me, it became easier to see a way out of it."

And she remembers the exact moment she started to feel like herself again. It was about six months after Chille's heart failure and two months after the end of her intensive therapy regimen. She heard a noise in his room in the middle of the night. Heart pounding, she hurried to check on him, but found him fast asleep.

"I thought, 'He looks so beautiful,'" she says. She got her camera and snapped a photo. "I realized that I was coming back to my old self. Only better and smarter."