PORTLAND (AP) — For years, Oregon has sat proudly atop national rankings for the smallest percentage of babies born at low birth weights; in 2000, just 5.6 percent of Oregon babies were born underweight, compared to 7.6 percent nationally.

But this year, the state has slipped to number three in the rankings, with 6.1 percent of babies statewide born underweight, according to a national report released Tuesday by the Baltimore-based Annie E. Casey Foundation.

The number likely reflects problems that pregnant women have been having in accessing prenatal care, said Tina Kotek, who directs Children First for Oregon.

“I know the state has been struggling to increase the number of women with access to prenatal health care,’’ said Kotek, who recently won the Democratic primary for a state House seat in a Portland district.

Oregon has been supplanted in the rankings by Alaska and Washington, which tied for the No. 1 spot with 6 percent of babies born underweight. At the other end of the rankings was Mississippi, which reported that 11.4 percent of babies born there were underweight. The national average is 7.9 percent.

Dr. Linda Wallen, the director of neonatology at Oregon Health & Science University, said some of the creep toward more underweight births could be due to the use of fertility drugs and treatments, which can result in multiple births. There’s also greater agreement among the medical community that babies who are born weighing as little as 4½ pounds have a good chance to survive, and even thrive, she said.

The findings were released as part of the Casey Foundation’s annual Kids Count report on the health and well-being of children and teens. The report measures each state’s progress in 10 categories, including infant mortality, poverty rates and single-parent families.

Oregon’s overall rank for child well-being was a relatively healthy 15, but that number can be deceptive. For instance, though the Casey Foundation credits the state with only a 6 percent high school dropout rate, a recent study by Education Week newspaper found that 30 percent of Oregon students will fail to graduate from high school after four years.

The discrepancies are due to the methods used to arrive at the eventual figure; the Education Week figures also do not include students who get a GED or attend alternative schools.

Kotek called the dropout figures, “one of the statistics that I am least comfortable with,’’ but pointed to others that she said accurately painted a picture of child welfare in the state, or lack thereof. For example, she said the foundation’s data show that 35 percent of Oregon children live in families where neither of their parents has full-time, year-round employment.

That underlines Oregon’s shift toward temporary, seasonal employment, she said, whether it’s in tourism on the Coast during winter or in construction jobs that stop when the snow begins.

Other key findings of the report:

— Mirroring the rest of the country, Oregon’s teen birth rate fell between 2000 and 2003, from 43 out of every 1,000 to 34.

— The percentage of children in single parent families fell between 2000 and 2004, to 29 percent from 32 percent.