Researchers in Boston tracked about 850 babies and their mothers over three years. Babies who were breastfed for at least four months had a one in 14 chance of being obese at age 3, regardless of when they first received solid food.
But, babies who were fed formula had a one in four chance of being obese at age 3, if they started eating solid foods before they were 4 months old.
If parents waited until between 4 and 5 months, the child's chance of being obese was one in 20.
The findings, published in the journal Pediatrics, support US guidelines that suggest parents wait until babies are between 4 and 6 months old to start feeding them solid foods.