Children of obese parents may be at risk for developmental delays, says the study, published in the Journal of Pediatrics.
In the United States, an estimated one in five women is obese when they get pregnant, registering a body mass index above 30. The healthy average is between 18.5 and 24.9.
But few studies have looked at the father's weight, even though 20% to 30% of US adults -- both male and female -- are obese.
A team from the Eunice Kennedy Shriver National Institute of Child Health and Human Development set out to learn more.
"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," said lead researcher Dr. Edwina Yeung, an investigator at the institute.
Effects differ based on the heavier parent
The researchers looked at 4,821 children from 4 months old to the age of 3, between 2008 and 2010, and found specific effects on children, depending on which parent is obese.
Compared with normal or underweight mothers, children of obese mothers were more likely to have difficulty using small muscles, such as those in their fingers or hands.
Paternal obesity was associated with increased risk of failing at personal-social activities, such as feeding themselves, playing and undressing themselves. Those born to extremely obese couples also were more likely to fail problem-solving tests.
The researchers used data from the ongoing Upstate KIDS study, which follows over 6,000 children born in New York state.
It aims to track the growth, motor and social development of children and its correlation with infertility treatments, obesity, rising maternal age and pregnancy complications.
Parents are asked to regularly complete the Ages and Stages Questionnaire for their children.
"The questionnaire is a screening tool. It's an indicator of whether a child is on track for behaviors appropriate to his or her age," Yeung explained.
The tool was designed to pinpoint development progress and catch delays in children up to the age of 6.
Yeung's team looked at reports recorded by parents at 4, 8, 12, 18, 24, 30 and 36 months. According to Yeung, although parents are self-reporting information, the potential for bias is small, as questions are straightforward (PDF).
"The fine motor skills part, for example, asks such questions as whether the child can turn the pages of a book or whether he or she can stack blocks," she explained.
Mothers also completed a questionnaire about health status and lifestyle at enrollment to determine both parents' height and weight before pregnancy.
According to the study authors, factors associated with obesity -- such as lower income and education, smoking and alcohol use -- were taken into to consideration when analyzing the results.
Weight at delivery was taken from electronic birth certificates.
Cause remains unclear
It is not unclear exactly why parental obesity might increase developmental delays in children.
"Our study wasn't designed to prove cause and effect. At this point, we only have correlations between parents' BMI and children's scores on a screening questionnaire," Yeung stressed.
However, the authors note that studies on animals show that obesity during pregnancy may promote inflammation, which can affect the fetal brain.
"Obesity is correlated with a rise in inflammation and in hormones that regulate body fat and metabolism. One theory is that these hormones might influence the development of the baby's brain," Yeung said.
Other theories are that high blood sugar or a shortage of certain nutrients might influence brain development.
The study offers less information on the potential effects of paternal obesity on child development, but the authors cite previous research hypothesizing that obesity could affect the expression of genes in sperm.
Scott Johnson, a professor in developmental psychology at the University of California, Los Angeles, was fascinated by team's results.
"The authors pose some interesting possible mechanisms by which maternal and paternal obesity could be responsible for the outcomes they reported," Johnson said.
He thinks the theories that maternal obesity poses a risk to a fetus' development are sound but says much more research is required.
"It wasn't clear to me how that general property induces specific deficits in fine motor skills," Johnson said.
The effects of obesity in male sperm are also not sufficiently explained, but the developmental psychology expert says it is not impossible.
"It's not a crazy idea. It has been speculated for some time that there may be distinct paternal genetic contributions to autism risk, for example," he said.
"Overall, the study has some important findings and an intriguing set of possible explanations," Johnson concluded.
However, at this point, nothing can be confirmed.
"It's really important to stress that these results need to be confirmed by other studies," Yeung agreed.
But that is not easy, as recruiting families and collecting and analyzing the data could take years. If the team's results are confirmed, physicians who work with young children might take parents' weight into account when screening for development.
In the end, it's one more reason to keep an eye on your waistline.
"We know that obesity greatly increases the risk for such conditions as heart disease, diabetes and stroke. For anyone, male or female, pregnant or not, it makes sense to attain a healthy weight," Yeung said.