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Teen pregnancies rose in the United States for the first time since 1991, the National Institutes of Health reported Friday.

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 Read full article »

CNN's Amy Burkholder contributed to this report.