Overweight preschoolers who keep their extra weight as they grow have a greater asthma risk at the age of seven, but the baby fat doesn't seem to matter for children who later slim down, according to a study.

Researchers writing in Pediatrics found that of more than 2,000 Swedish children they followed to the age of eight, those who were overweight or obese at age seven were more likely to have asthma than their thinner peers.

By contrast, children who were heavy as toddlers or at age four, but not at age seven, were no more prone to asthma than children who'd always been of normal weight.

"High body mass index (BMI) during the first 4 years does not increase the risk of asthma at school age among children who have developed a normal weight by age seven," wrote Jessica Magnusson, at the Karolinska Institute in Stockholm, who led the study.

"However, high BMI at age seven years is associated with an increased risk of asthma and sensitization to inhalant allergens."

Body mass index is a measure of weight relative to height.

A number of studies have found that heavy children have a higher risk of asthma, or more severe symptoms, but whether the extra pounds are the cause remains unclear.

"We don't think we can say that overweight is causally associated with asthma -- that is, that overweight causes asthma," Magnusson said.

Overall, six percent of the eight-year-olds followed in the study had asthma, while 10 percent of the children who were overweight at age seven did.

The researchers considered other factors, such as the parents' history of allergies and whether a mother smoked during pregnancy. They found that being overweight at age seven was linked to a doubling in the risk of asthma.

That was true of seven-year-olds who'd been of normal weight earlier in life, as well as those who'd been heavy at age four.

At any age, there were about 300 children of the 2,000 who were overweight, but fewer were persistently heavy. Only some 122 remained overweight from the age of one to age seven.

So parents should feel reassured, Magnusson said, that those early extra pounds often do not last -- and that children whose weight normalizes may not have an increased asthma risk.