Half of all booster seats don’t ensure that adult-sized seat belts fit children properly in all vehicles, according to an analysis released Thursday by the Insurance Institute for Highway Safety.

Of 83 booster seats evaluated, 41 fell into the “check fit” category, meaning they might not provide adequate seat-belt fit for all children in all vehicles, the study found. Parents with those seats should check that the booster makes the lap belt lie flat across the child’s upper thighs and the shoulder belt fit snugly across the middle of the shoulder, researchers said. If it doesn’t do both, they said, parents should use another booster seat. The analysis covered all U.S. booster seats being manufactured.

Because vehicle seat belts are anchored in different locations, the researchers said, the same booster might properly position the seat belt in a grandparent’s sedan but not in a family’s minivan. The study covered 62 models but 83 seats because 21 models were tested in both full-back and backless positions. Some models ended up in two different categories because their full-back position better guided the shoulder belt to a proper fit, researchers said. The rankings are available at www.iihs.org.

Parents who focus on fabric colors and cup holders when shopping for a booster should hone in first on how it positions the seat belt in every vehicle in which the child will ride, experts say.

“We still have a lot of boosters out there not always doing a good job in every vehicle,” said Anne McCartt, senior vice president for research at the Insurance Institute for Highway Safety, an Arlington-based nonprofit organization funded by auto insurance companies.

Thirty-one seats, the most in the institute’s four years of analyzing boosters, fell in the “best bets” category. That means they properly position the seat belt in all vehicles. Even with those boosters, McCartt said, “it doesn’t hurt to check.”

Five boosters were ranked “good bets” for providing a proper seat-belt fit in most vehicles. Six were listed as “not recommended.”

Infant seats and convertible car seats restrain babies and younger children with their own five-point harnesses. Boosters are designed to elevate children typically ages 4 to 8 to help seat belts, which are designed to restrain adults, fit children’s smaller frame correctly. A child can suffer serious injuries to the hips, spine and internal organs in a crash — a problem known as “seat belt syndrome” — when a lap belt rides too high or the shoulder belt doesn’t properly restrain the torso.

The good news: Boosters are getting better. In 2008, the institute’s first year of analyzing booster seat-belt fit, 10 seats made the “best bets” list. That number has grown as manufacturers have made design changes, McCartt said.

Like most states, Maryland, Virginia and the District require children to be “properly restrained” in a harnessed car seat or booster up to age 8 and then with a seat belt. Maryland law also allows children to graduate from child restraints after reaching 4 feet 9 or 65 pounds.

McCartt said she’s concerned that state laws encourage some children to move on from boosters too soon. She advised parents to use age and height requirements as a general guide but to keep children in a booster as long as necessary to ensure a proper seat-belt fit.

“Many children older than 8 belong in a booster,” she said.