NEW YORK — The percentage of underweight babies born in the United States has increased to its highest rate in 40 years, according to a new report that also documents a recent rise in the number of children living in poverty.

The data on low birth weights is worrisome because such babies — those born at less than 5.5 pounds — are at greater risk of dying in infancy or experiencing long-term disabilities.

The findings were released today in the annual Kids Count report on the health and well-being of America's youth, which measures the states in 10 categories. Overall, the report found progress, as well as some setbacks.

"Well-being indicators have largely gotten better for teens, and they've gotten worse for babies," said Laura Beavers, coordinator of the Kids Count project for the Baltimore-based Annie E. Casey Foundation.

The report documented improvements in the child death rate, teen death rate, teen birth rate, high school dropout rate and teens not in school and not working. There was no change in the infant mortality rate, while four areas worsened: low-birthweight babies, children living with jobless or underemployed parents, children in poverty and children in single-parent families.

In composite rankings for all 10 indicators, New Hampshire, Minnesota, Massachusetts, Connecticut and Utah ranked the highest.

Hawai'i was ranked 13th. Hawai'i also achieved the nation's lowest teen death rate.

The lowest-ranked state overall was Mississippi; others at the bottom of the list were Louisiana, New Mexico, Alabama and South Carolina.

Beavers noted that in many categories, the United States compares poorly with other developed countries. A recent study released by UNICEF ranked the U.S. second worst out of 33 industrialized nations in a composite index on child well-being, and it was 29th in regard to the percentage of babies with low birth weights.

RISE IN MULTIPLE BIRTHS

According to Kids Count, the latest available federal data, from 2005, showed that 8.2 percent of U.S. babies were born at low birth weight, a level not seen since 1968.

The worst rate — 11.8 percent in Mississippi — was nearly twice the 6.1 percent rate in the best states: Alaska, Oregon and Washington.

Beavers said part of the overall increase in low birth-weight babies was due to a rise in multiple births as more older women use fertility treatments to conceive. But she said the birth-weight problem also has been worsening for single-baby deliveries.

PREMATURE DELIVERIES

The rate of low-weight births is sharply higher for blacks (13.6 percent) than whites (7.3 percent) or Hispanics (6.9 percent). One important factor, Beavers said, is a woman's overall health at the time of pregnancy and her access to good prenatal care.

Dr. Alan Fleischman, medical director of the March of Dimes, said the increase in underweight newborns is closely linked to a rise in premature births.

He agreed with Beavers that better socioeconomic conditions for pregnant women would help. But Fleischman also said the U.S. medical profession should be more rigorous in encouraging women to continue their pregnancies as close to term as feasible, and reduce the number of early, induced deliveries, often Caesarean, that frequently produce underweight infants.

Regarding its key economic indicator, the Kids Count report said 18 percent of U.S. children — 13.3 million of them — were living in poverty in 2006, up by 1 million children from the 17 percent rate in 2000. It said child poverty increased in 32 states during that period.

"It's disconcerting, because between 2000 and 2006 the economy was doing pretty well," said Beavers, who predicted the poverty rates would worsen amid the current economic downturn.

The report's data was based on the official poverty measure as determined by the Office of Management and Budget. Its 2006 poverty line was $20,444 for a family of two adults and two children.

POVERTY IN MISSISSIPPI

Among the states, the child poverty rate ranged from a low of 10 percent in Maryland and New Hampshire to a high of 30 percent in Mississippi. The national rate was 11 percent for white children, 36 percent for blacks and American Indians, and 28 percent for Hispanics.

In a separate ranking of 50 large cities, the lowest child poverty rate — 10 percent — was in Virginia Beach, Va. The highest, 44 percent, was in Detroit.

BEST AND WORST STATES FOR KIDS

The Annie E. Casey Foundation ranks every state each year on 10 health indicators for children. Here are the indicators with the states that were best and worst for each one: