After years of blaming overweight mothers for putting their children’s health at risk a new study is pointing a finger at obese fathers for preschoolers who have trouble fitting in.

According to researchers, the social problems might include having difficulty relating to others, and other delays in “personal-social” functioning.

“The previous U.S. studies in this area have focused on the mothers’ pre- and post-pregnancy weight,” first author, Edwina Yeung, of the U.S. National Institutes of Health, said in a statement.

“Our study is one of the few that also includes information about fathers, and our results suggest that dad’s weight also has significant influence on child development.”

The paper, published in the journal Pediatrics, is likely to add to emerging concerns that obesity somehow alters a man’s sperm, leading to downstream effects on his baby’s brain development.

Investigators also found that when mothers were obese, their toddlers were more likely to fail tests of fine motor skills.

The report comes as rates of overweight and obesity soar among a new generation of expectant mothers. In the U.S., one in five women are obese when they begin their pregnancies. In Canada, 23 per cent of women of childbearing age are obese.

In the past year alone, studies have linked maternal obesity with higher odds of autism and a shorter life expectancy in children, while a Quebec team found plus-sized pregnancies may hike a baby’s lifetime risk of stroke and heart attack.

But every child is a product of two parents, Yeung said. Given that, “it makes sense to look at each parent,” she said in an email interview.

Her team’s study is based on data collected from a New York State study involving more than 5,000 women who gave birth between 2008 and 2010.

The women provided information on both partners’ height, weight, health and lifestyle.

The parents also completed the “Ages and Stages” questionnaire, a screening tool used to determine if a child is on track for behaviours appropriate to his or her age. Children were tested starting at four months, and then six more times, through age three.

The screening test focuses on the child’s “solitary social play and play with toys and children.” Sample questions ask if a 16-month-old can turn the pages of a book, or stack three small blocks on top of each other. The 36-month questionnaire asks, among other things, whether the child, when looking in a mirror and asked, “Who is in the mirror?” says either “me” or his or her own name, or if the child takes turns waiting while another child takes a turn.

Compared with the offspring of normal or underweight mothers, children of obese mothers had increased odds of failing the fine-motor skills tests.

Children of obese dads, meanwhile, were 75 per cent more likely to fail the “personal-social” sections compared with children of normal weight fathers.

It’s important to keep in mind that some delays may not be permanent and some children may simply outgrow them.

When both parents were obese, children were almost three times more likely to fail the test’s problem-solving questions by age three. (Sample questions include, “When you say, ‘Say ‘seven three,’ does the child repeat just the two numbers in the same order? Or, “If your child wants something he cannot reach, does he find a chair or box to stand on to reach it?”)

According to the researchers, “the findings suggest that maternal and paternal obesity are each associated with specific delays in early childhood development.”

The study doesn’t prove cause and effect. And investigators didn’t test the children directly for specific disorders. The questionnaire is just a screening, and not diagnostic, tool.

However, earlier studies have linked obesity in mothers with an inflammation in the fetus’s developing brain, while last year researchers reported that sperm from obese men appears to differ from that of normal weight males. Obesity seems to bring on mutations and “epigenetic” changes in sperm. Epigenetics is the study of how genes are switched on or off in response to environmental conditions — in this case perhaps, the father’s weight and diet.

Differences were found in hundreds of genes, including those associated with brain development as well as FTO — the leading “fat” gene with the strongest link to obesity. That may explain why children born to obese men are more likely to grow into obese children and adults themselves. (The same is true for children of obese mothers).

Yeung said doctors might want to take parental weight into account when screening children for potential developmental delays.

“It’s important to keep in mind that some delays may not be permanent and some children may simply outgrow them,” she said.

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