Formula-fed babies who start eating solid foods before they are four months old may be more likely to become obese than those who start later, a new study suggests.

The study, which appears in the journal Pediatrics, tracked about 850 Massachusetts babies and their mothers over three years.

The researchers recruited the mothers when they were pregnant, gave them regular questionnaires and met with them in person to gather further data.

They found that for babies who were breastfed for at least four months, the age when they first received solid food – either before four months, at four or five months, or at six months or later - had no effect on whether they became obese by three years old.

Overall, the breastfed babies in the study had a one in 14 chance of being obese as preschoolers.

But babies who were formula-fed from the start or moved to formula before four months had a startling one in four chance of being obese at age three if they started eating solid foods before they were four months old.

If parents waited until between four and five months, the kids' chances of being obese were one in 20.

Strangely, the chance of being obese increased again if babies didn't start eating solid foods until they were at least six months old. But the study had so few babies in that category that the authors couldn't make a firm conclusion about the risk of waiting longer to feed a baby solid foods.

The study should help clear up some questions about the best time to transition to solid foods, as previous studies have shown conflicting results on the matter, the study authors point out.

It's not clear why some parents choose to move their babies into solid foods early. One reason could be that baby formula is expensive compared to baby cereals and other solid foods.

Some parents may also believe that feeding solids early can help an infant sleep better, although there is no scientific evidence for that.

While guidelines in the U.S. suggest starting solid foods between four and six months, the guidelines from the Canadian Paediatric Society call for parents to wait until babies are six months old before introducing solids.

"The Canadian Paediatric Society recommends exclusive breastfeeding for the first six months of life for healthy, term infants," the agency says on its website.

"…Nutrient-rich complementary foods, with particular attention to iron, should be introduced at six months."