When it comes to children's shoes, the size listed by the manufacturer is rarely the true size, new research indicates. In nearly all cases, the manufacturers overstate the size, according to findings presented this week at the American Academy of Orthopedic Surgeons (AAOS) annual meeting in Las Vegas.

"The most striking finding of our study was that the majority of outdoor shoes and slippers of children were too small," study chief Dr. Norman Espinosa, from the University of Zurich, Switzerland, told Reuters Health. "Interestingly, the shoe sizes given by the manufacturers almost never matched with the true sizes measured by us."

Children wearing shoes that are too small may be at risk for developing foot deformities, the researchers warn.

Another interesting finding, he added, is that the prevalence of hallux valgus deformity among the children studied was higher than previously reported. Hallux valgus is a condition that occurs when the big toe begins to angle sideways, toward the second toe, causing a bump on the side of the foot just below the big toe, the AAOS explains. This bump is called a bunion, and can become swollen and painful. Shoes that are too tight are believed to be one of the leading causes of this condition.

The results stem from a study of 248 school children from an area in Switzerland who had the inner length of their shoes, the length of their feet, and the hallus valgus angle measured.

The study found that most of the children tested where wearing the wrong shoe size.

For outdoor shoes, only about 34 percent fit perfectly, about 53 percent were too small, and 13 percent were too big. For indoor shoes/slippers, 28 percent fit perfectly, 62 percent were too small and roughly 10 percent were too big.

Just 7.6 percent of outdoor shoes had a correctly stated shoe size. For 90 percent of shoes, the actual size was smaller than that listed by the manufacturer. The results were even more striking for slippers: only 2.4 percent had a correctly stated size and 97.6 percent were too small.

"In our study...more than 90 percent of both outdoor and indoor shoes/slippers worn by the children were too small," Espinosa noted in a statement.

Overall, 3.3 percent of children had an abnormal hallux valgus angle.

He advises parents to measure their children's feet every time they purchase new footwear, and they should consider the actual size of the shoe rather than just the number marked on the inside of the shoe or the box. "We truly did not expect such a large percentage of incorrectly declared shoe sizes," he said.

Espinosa also suggests parents check for shoe fit every month or so, especially during times of a growth spurt. Many children will often outgrow their shoes well before the shoes are worn out.