ATLANTA, Georgia (CNN) -- Teen pregnancies rose in the United States for the first time since 1991, the National Institutes of Health reported Friday.
Pregnant teens are less likely to get prenatal care and gain appropriate weight, experts say.
The new data also show that eighth-graders smoke less, according to the report "America's Children in Brief: Key National Indicators of Well-Being 2008."
The report comes after a spate of high-profile teen pregnancies: that of 17-year-old TV star Jamie Lynn Spears, who recently gave birth to a daughter, as well as the pregnancies of numerous students at Gloucester High School in Massachusetts.
Federal health experts said they don't know why the teen pregnancy numbers went up from 2005 to 2006, and that not enough data have been collected to say whether it's a trend.
"It may be a blip in the data, and it may come down," Edward J. Sondik, Director of the National Center for Health Statistics in the Centers for Disease Control and Prevention, said.
Among other key findings from the study: Injury and mortality among adolescents ages 15 to 19 went down from 2004 to 2005. But more youth offenders ages 12 to 17 were involved in serious violent crimes in the same time period. The number of students who reported using illicit drugs over the past 30 days did not change significantly from 2006 to 2007 among eighth-, 10th-, or 12th-graders.
Read more about teenage behavior in the report »
Pregnant teens aged 15 to 19 are less likely to get prenatal care and gain appropriate weight, experts say. They are also more likely smoke than pregnant women aged 20 years or older.
Teen pregnancy is "one of the key indicators for the health of the teen population because it not only reflects their health at this point, but it reflects their health and well-being for the next 20 to 40 years," Sondik said.
The numbers also say something about the health of these teenagers' children, who are more likely to have a low birth weight, said Sondik, which is a "cause of concern."
Low birth weight infants, defined as less than 5 pounds 8 ounces, are at increased risk for infant death and such lifelong disabilities as blindness, deafness and cerebral palsy. The report also showed an overall increase in low birth weight infants.
In 2005, the number of births for girls aged 15 to 17 was about 133,000, or 21 for every 1,000 girls. That number rose to nearly 139,000, or 22 for every 1,000 girls, in 2006.
Along the same lines, 1/3 of girls in the United States got pregnant before age 20, and more than 435,000 babies were born to teens between 15 and 19 years in 2006, according to the Centers for Disease Control and Prevention.
A cutback in community resources for youth over the last eight years could help explain the increase in teen pregnancies, said Michele Ozumba, director of the Georgia Campaign for Adolescent Pregnancy Prevention.
"All small-community based organizations doing youth programming are struggling just to keep the doors open," she said. "There are no additional resources to respond to the needs that we're seeing every day."
Still, she cautioned that the data reflect only one year of change, meaning it does not necessarily point to a trend.
The report also showed that daily smoking among eighth-graders in the U.S. went down from 4 percent in 2006 to 3 percent in 2007. That's a striking decrease from 1996, when 10 percent of eighth-graders smoked daily.
"They're making right choice in early lives, and we certainly hope that this trend will remain," Sondik said.
He attributed the downward trend to efforts convincing kids and adults not to smoke, as well as policies that restrict smoking in public places and tax cigarettes.
The report noted that 9 percent of children have asthma in the general population, but among specific groups, asthma affects 13 percent of black children and 26 percent of Puerto Rican children.
"We hope that policy-makers will turn their efforts to try to understand more what these differences are due to and reduce them," Sondik said.
CNN's Amy Burkholder contributed to this report.