CHICAGO – Many young children are too heavy for standard car-safety seats, and manufacturers are starting to make heftier models to accommodate them, according to research on the obesity epidemic’s widening impact.

More than a quarter of a million U.S. children ages 1 to 6 are heavier than the weight limits for standard car seats, and most are 3-year-olds who weigh more than 40 pounds, the study found.

Unless exceptionally tall, a 3-year-old weighing more than 40 pounds would generally be considered overweight.

Lead author Lara Trifiletti said researchers at a safety center at Johns Hopkins Hospital became interested in the topic because they saw children “who were very obese and our car-seat technicians were having a hard time finding car seats to fit them.”

She did the research at Johns Hopkins but now works at Ohio State University’s Children’s Research Institute.

Using inadequate car seats for heavy children could put them at increased risk for injury in a car accident, the researchers said.

“We don’t recommend that a parent use a restraint system for a child that has outgrown that system,” said Eric Bolton, a spokesman for the National Highway Traffic Safety Administration. “It is risky.”

Based on national growth charts and the 2000 Census, at least 283,305 children ages 1 to 6 are too heavy for standard safety seats. That includes nearly 190,000, or almost 5 percent, of U.S. 3-year-olds, the researchers said.

Trifiletti said the phenomenon mostly affects youngsters whose weight exceeds the limits of standard seats with built-in safety harnesses, which are designed for 1-to-3-year-olds whose weight does not exceed 40 pounds.