The research also reveals some surprises: About four out of five children with insurance coverage gaps have parents who work; two-thirds of them live with both parents; and more than half are white.
At least 9 million U.S. children, or about 12 percent, lack health insurance, based on a federal survey in 2003.
Researchers who produced the latest study say that number is likely higher because many kids who lack health insurance during part of their childhood aren't included in that number.
"There is an oversimplified view of what is uninsured," said Lynn Olson of the American Academy of Pediatrics, who led the study.
Many studies have documented the health barriers faced by uninsured children, such as missing regular checkups and visiting hospital emergency rooms for routine care.
"It really calls attention to an important group of children we often don't consider," said Dr. Glenn Flores, director of the Center for the Advancement of Underserved Children at the Medical College of Wisconsin, who was not connected to the research.
Marian Blackmon, a telephone operator in Jackson, Miss., said that when her husband retired last year, her three sons were dropped from his health plan.
Blackmon recalled how she avoided taking one of her 6-year-old twins to the pediatrician when he became feverish because she did not want to pay out-of-pocket for the visit.
They estimated that almost 7 percent of children were uninsured, but another 8 percent lacked coverage for part of the year.
The researchers found that the children with intermittent coverage were more likely to postpone medical care than uninsured kids.
For example, 20 percent of children who were uninsured for part of the year delayed getting medical attention because parents worried about cost compared to 16 percent without insurance and 4 percent with public and private coverage.