Perhaps not surprisingly, children whose mothers gained too much weight during pregnancy tend to have more body fat than those whose moms stayed within the recommended weight-gain range, a new study finds.

The findings support the latest pregnancy weight-gain recommendations from the U.S. Institute of Medicine (IOM), according to senior researcher Dr. Sian Robinson, a nutritionist with the MRC Epidemiology Research Centre in Southampton: 25-35 pounds for normal-weight women, 15-25 pounds for overweight women, and 28-40 pounds for underweight women.

UK researchers found that among 948 children followed to age 6, those whose mothers gained more than the recommended amount of weight during pregnancy had a greater amount of body fat, on average.

Compared with their peers whose mothers gained within the recommended range, these children had 10 percent more fat mass at the age of 6, according to findings published in the American Journal of Clinical Nutrition.

The researchers stress that the study does not prove that mothers’ extra pregnancy pounds, per se, cause their children to put on more body fat.

However, the results add to evidence linking moms’ pregnancy weight gain to their kids’ odds of becoming overweight.

For their study, Robinson and her colleagues used an X-ray technique called dual X-ray absorptiometry to measure body composition in 948 children at birth and ages 4 and 6. (Each X-ray test exposed the children to the equivalent of one to two days’ worth of normal environmental radiation.)

Half of the mothers in the study gained more weight during pregnancy than recommended by the IOM. On average, their children had more body fat at each age than those born to mothers whose weight gain fell within the IOM recommendations.

The relationship between pregnancy weight gain and children’s body fat at age 6 remained after the researchers factored in a number of other variables - including children’s birth weight, mothers’ education levels and whether and how long the children breastfed. The link with children’s body fat levels at age 4, however, was no longer significant.

“As our data are observational we cannot infer that excess weight gain causes differences in body composition in the children,” Robinson told Reuters Health in an email. There are other factors not measured in the study - such as the children’s diets and exercise habits - that might at least partly account for the link between pregnancy weight gain and children’s body fat levels, according to the researchers.

However, Robinson said, it is biologically plausible that excess pregnancy pounds would affect children’s fat accumulation.

Animal research suggests that the fetal environment, including a mother’s diet, may help “program” a child’s future appetite regulation, metabolism and predisposition to weight gain - though, Robinson added, “we currently know little about these effects in humans.”

Also unclear is the significance of the extra fat mass seen in 6-year- olds whose mothers gained more than the IOM recommends. Robinson said she and her colleagues are continuing to follow the children to see whether that excess body fat has any health implications.

But if gaining too much weight during pregnancy promotes childhood fat gain, inadequate pregnancy weight gain may have similar effects. Robinson’s team found evidence, albeit weaker, that children also had more body fat at the ages of 4 and 6 when their mothers fell short of the IOM recommendations, as compared with adequate weight gain.

Lab research, the investigators note, suggests that gaining too little weight during pregnancy may lead to childhood fat gain by affecting the way the nervous system regulates appetite.

SOURCE: http://www.ajcn.org/cgi/content/abstract/ajcn.2009.29128v1 American Journal of Clinical Nutrition, online April 7, 2010.