(CNN) The percentage of younger children and teens hospitalized for suicidal thoughts or actions in the United States doubled over nearly a decade, according to new research that will be presented Sunday at the 2017 Pediatric Academic Societies Meeting.

A steady increase in admissions due to suicidality and serious self-harm occurred at 32 children's hospitals across the nation from 2008 through 2015, the researchers found. The children studied were between the ages of 5 and 17, and although all age groups showed increases, the largest uptick was seen among teen girls.

"We noticed over the last two, three years that an increasing number of our hospital beds are not being used for kids with pneumonia or diabetes; they were being used for kids awaiting placement because they were suicidal," said Dr. Gregory Plemmons , presenter of the study and an associate professor of pediatrics at Monroe Carell Jr. Children's Hospital at Vanderbilt.

Sensing a trend, Plemmons and his colleagues conducted the research to see what was happening across the country, he said. "And it confirmed what we were feeling: that the rates have doubled over the last decade."

Peaks in fall and spring

Plemmons looked at administrative data from 32 children's hospitals to identify the total number of emergency department and inpatient visits over eight years ending in 2015. He found 118,363 children between the ages of 5 and 17 with a discharge diagnosis of suicidality or serious self-harm.

"We didn't look at completed suicides, and we didn't look at actual numbers of total suicides. All we actually could look at were those kids that were admitted to a children's hospital with a diagnosis of suicide ideation or a suicide attempt," Plemmons said.

Slightly more than half, 59,631 children, were between the ages 15 and 17, and nearly 37% were between 12 and 14. Children 5 through 11 -- a total of 15,050 kids -- represented nearly 13% of the total.

Increasing suicide rates among children mirror adult numbers, Plemmons said. Children's numbers more than doubled over the study period, increasing from 0.67% of children admitted to hospitals in 2008 to 1.79% in 2015. Annually, the 15-to-17 age group averaged an increase of 0.27%, the 12-to-14 age group averaged 0.25%, and the-5 to-11 age group averaged 0.02%.

In 2008, about 60% of all children and teens hospitalized as a result of suicidal thoughts or attempts were girls, and, by 2015, that number had increased to 66%, said Plemmons. While he did not break down age groups, he said the Centers for Disease Control and Prevention reported that suicides had tripled in girls , ages 10 through 14, between 1999 and 2014.

"Females are more likely to attempt, but males in general are more likely to succeed," said Plemmons.

Plemmons noted one "really interesting" finding he has not seen elsewhere.

"When we looked at the number of kids awaiting placement or admitted at one time, month by month, there is a huge difference in the months," he said. "Certainly, the month of the year that is the lowest for suicidal thoughts and ideation is July. And we see those numbers creep back up right when school starts."

Peaks can be seen in the fall and the spring, with a lull in the summer.

The reason children think about or attempt suicide is the "million-dollar question," Plemmons said. "Family history of depression or suicide, family violence, child abuse, gay and lesbian youth, history of bullying -- those are all risk factors that have been reported. We didn't look at any of those specific factors in our study."

School drives a lot of this behavior, he said, and he wonders whether it might represent the overall influence of "social media and socialization in general. ... We know that school's a stress just like a job is a stress, so it may just be that removal of that situation allows some kids to cope a little bit better."

One of the predominant theories behind teen suicide is cyberbullying, Plemmons said. "It's anonymous today, I think that's a big difference. Years ago, you knew who (the bullies) were."

Additionally, girls are entering puberty about a year earlier "than they historically have, and puberty in itself is a risk factor for suicide."

A lack of coping skills?

Avital K. Cohen, a licensed psychologist who was not involved in the research, believes that a variety of factors contribute to the rising trend seen in this study.

"Our expectations of children have changed pretty significantly in the last several decades," Cohen said, although she doesn't have research to support her opinion.

"Many parents try to protect their children from experiencing failure when they are young; thus, when they experience it later in life, they may not have developed the resources and/or coping skills they need to manage it," she said. A bigger emotional response is triggered, which "might contribute to increases in suicidal expression."

She also believes that social media contributes to an increase in anxiety and depression, and cyberbullying is a phenomenon "that I don't know that any of us fully understand." It's unclear what the long-term implications might be when kids have grown up in a world of social media that begins when their parents post pictures and videos of them as infants and toddlers, she said.

David Palmiter , a professor of psychology at Marywood University in Scranton, Pennsylvania, said "there's kind of a deeper issue here that most of the kids that need mental health care don't get it."

"Approximately 20% to a third of the kids who need care get it, but the large share don't," said Palmiter, who also was not involved in the research. "And those who do get care, usually they have had to suffer for a few years before they get it, and often, the care is not necessarily evidence-based."

Even though the percentage of suicidal kids may be "alarming," the study doesn't prove that rates of suicidality are on the rise, Palmiter said. "To do that, you need more research."

Still, he believes the research is important in that it suggests that efforts to educate the public about identifying problems are paying off, with parents and teachers recognizing kids who need help.

"If anything, parents have to get better at recognizing the symptoms of depression, the lack of joy, the concentration problems, the isolating behaviors that occur," Palmiter said.

"Historically, we've wrongfully thought that suicidality becomes an issue in adolescence, and we haven't really looked at younger children so much," he said. "And we now know that's not right. You can have kids under the age of 12, under the age of 10, thinking about attempting or even completing suicide."

The youngest suicides

Carl Tishler, an adjunct associate professor of psychology and psychiatry at The Ohio State University, said that when it comes to suicidality, there are significant differences between teens and those younger than 11.

"The younger the child, typically, the more disturbed the child is," said Tishler, who did not participate in the research. "With these little kids, a lot of times, they come into the emergency room, and it looks like an accident."

Children under 6 who complete or attempt suicide are "pretty damaged kids," he said, explaining that the "one thing that jumps out" in case studies of these young children is how "they've been dramatically exposed to domestic violence."

"They will do things like jump out of window; they will jump off a balcony; they will run into traffic. One of the cases actually was a little kid who dressed up and was playing Superboy or Superman and managed to hang himself on a window cord." Among children between 7 and 11, hangings or suffocations are more likely, he said.

"When you look at it on the outside, it looks like a normal child playing and jumped off his bed and he hurt himself," he said. Dig a little deeper, and "you find out the kid's father died of suicide" or that some other serious family trauma occurred. "It was definitely a suicide attempt, and the kid wasn't able to verbalize that they wanted to die."

"Some of these little kids will verbalize they want to die, that they don't want to live anymore, or they want to go to sleep and not wake up and stuff like that," he said.

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Health care providers staffing emergency departments are more sophisticated now, Tishler said, so they are asking questions and looking for chidlren who are attempting suicide.

"You just don't know. You have to be pretty tuned in to what you're dealing with and not say, 'Oh, a child under age 6 doesn't understand the permanence of death,' " he said, paraphrasing an "old theory" that children never attempt suicide.

Tishler also said that with "an estimated 12 to 15 million children on psychotropic medications," any time doses begin or get increased or decreased, "it may cause a change in emotional state which may reduce impulse control." This could contribute to suicidal urges.