Moms whose toddlers were overweight also reported being more satisfied with their children's size, possibly reflecting the belief that heavier toddlers are normal.
Mothers whose toddlers were underweight, on the other hand, had accurate perceptions of their child's size, but believed their children needed to be bigger, the study found.
The researchers asked 281 low-income, mostly African-American mothers of toddlers between 13 and 30 months of age to select a silhouette that they believed most accurately represented their child's shape. Mothers of overweight toddlers were 88 percent less likely to choose an accurate shape.
The authors had hypothesized before the study that low education level and low income, as well as being African American, were factors associated with inaccurate views of toddler weight.
"We live in this culture where people perceive overweight or chubby toddlers to be healthy infants or toddlers, and that's been a social norm," said Erin Hager, the study's lead author and an assistant professor at the University of Maryland School of Medicine in Baltimore. "We also live in a culture where there are so many overweight kids, so the overweight body type is becoming the norm."
Parents might idealize their child as being a normal size, the authors wrote, because a heavy child is sometimes seen as a "sign of successful parenting, especially during the early years when parents are responsible for their child's health, nutrition and activity opportunities," the authors wrote.
But an overweight child runs the risk of numerous health problems that can persist throughout life - from diabetes to heart disease - which is why Hager said it's important for parents to become more aware of what an overweight body type is.
"The best way is though pediatric visits," she said. "If people go to these well-child visits and we can get this dialogue going in a clinical setting about what a child's body proportion is, parents can start to visualize what the ideal proportion is."
She also said pediatricians often plot weight and height based on a child's age separately, but in order to get a true sense of overall proportion, they should be plotted together starting in infancy.
In an accompanying editorial, Dr. Eliana Perrin, an associate professor of pediatrics at the University of North Carolina School of Medicine at Chapel Hill, wrote that pediatricians should screen a child's body mass index (BMI) with parents starting at age 2 and discuss it in a culturally sensitive manner with parents, along with dietary and activity recommendations.
"Our own research revealed that more than 75 percent of parents of overweight children report never having heard that their children were overweight from their doctors, with worse rates for younger children," she wrote.