Mothers of overweight toddlers often mistakenly think their children are normal weight, and mothers of underweight toddlers often wish they were plumper.
The findings, based on a study of poor women in Baltimore, suggest that US mothers often do not have a realistic idea of their offspring's weight, and many still cling to the notion that a chubby child is healthy child.
"A long time ago, it was okay to value a chubby baby when kids were underweight and we had potato famines and what not. It was a sign you're doing well for yourself," said Erin Hager of the University of Maryland School of Medicine.
"But that is not how it is today in the United States," said Hager, whose study appears in the journal Archives of Paediatrics & Adolescent Medicine.
Hager works with low-income mothers in clinics in the Baltimore area to find ways to help prevent obesity, a condition that now affects about 17 per cent, or 12.5 million, American children and adolescents ages two through 19.
Poor children are hit especially hard. As many as one in seven low income preschool-age children are considered obese, according to the US Centres for Disease Control.
The researchers wanted to get an idea of how moms see their children, and how that might affect they way they feed them. For the study, they used a cartoon drawing of seven diaper-clad toddlers arranged in a row from underweight to obese.
They enrolled some 280 women aged 18 to 46 in the study, 72 per cent of whom were overweight or obese themselves. And they asked the moms to point to the picture that best resembled the height and weight of their own child.
The team already had information on the actual height and weight of the children, and they compared it to the drawing selections of the mothers.
Nearly 70 per cent of mothers were wrong in assessing their toddler's body size, and overweight mothers were the least likely to get it right.
"Specifically, mothers of overweight toddlers consistently tended to chose a silhouette that was smaller than their child's true body size," Hager said.
Temptation to overfeed
The team also asked the mothers to choose a picture they would most like their children to look like.
Most of the mothers - 71.5 per cent - were very satisfied with their toddlers' body size, and mothers of overweight toddlers were the most satisfied, the researchers found.
Among the moms who were dissatisfied, the team found more than 20 per cent of mothers of healthy weight toddlers and many of the mothers of the overweight toddlers wanted their kids to be bigger, Hager said.
"That suggests we may have a lot of parents who are trying to fatten up their babies," said Dr Eliana Perrin of the University of North Carolina at Chapel Hill School of Medicine, who wrote a commentary on the research in the same journal.
Doctors say the obesity epidemic may be eroding a general impression among the public of what healthy looks like in a toddler, and that suggests pediatricians need to be much more candid with parents about their child's weight.
The concern among scientists is that children's eating habits are shaped when they are very young, said Dr Stephen Cook, a member of the Executive Committee of the Section on Obesity for the American Academy of Pediatrics and associate professor of paediatrics at the University of Rochester Medical Centre.
"Kids who gain weight as toddlers tend to hold onto weight longer and tend to be overweight and obese in adolescence and adulthood," said Cook, who conducted a similar study in older children.
The CDC estimates that nearly one quarter of the four million births each year in the United States involve obese women. Obesity raises the risk for diabetes, hypertension, heart disease, osteoarthritis, stroke, gallbladder disease, sleep apnea, respiratory problems and even some cancers.