Girl babies born more than a week past term could be 12 percent more likely to grow up obese, according to a new international study led by a Liggins Institute researcher.

The study found that women born very post-term, at 43 weeks or more, were on average 1 kg heavier and had body mass index (BMI) 0.3 kg/m2 greater than women born at 38-40 weeks.

Researchers from the Liggins Institute at the University of Auckland and Uppsala University in Sweden crunched data from more than 200,000 Swedish women aged over 18 years, collected between 1991 and 2009.

“While the effect was relatively small, it’s an important part of the puzzle in explaining how early experiences help programme our metabolism and set our likely weight-range as adults,” says study lead Liggins senior research fellow Dr José Derraik.

“It’s possible that babies born post-term may be exposed to stress because of an abnormally long pregnancy or deterioration of the placenta that occurs late in pregnancy,” says Dr Derraik.

“We also know that an individual born post term is more likely to have a sibling or parent born post-term, suggesting genetics may also be important.”

The Swedish records didn’t include the weight of the women’s mothers, so the researchers were unable to rule out the possibility that women born post-term were heavier in part because their mothers were heavier.

But an earlier Liggins Institute study of children born post-term did factor in mothers’ weight and still found similar effects.

"We showed that children who were born post-term had more body fat and more abdominal fat, even after controlling for parents' BMI,” says Dr Derraik. “And their bodies were less efficient at handling sugar. All of these are risk factors for type 2 diabetes and cardiovascular disease.

"The study of Swedish women indicates these heightened risks may persist into adulthood, even though the effects were not as marked as those we observed in children,” he says.

"Our findings empower those who were born post-term to make better lifestyle choices, such as improving their diet and exercising more, to minimise their risks of developing diabetes or heart disease.”

In New Zealand, an estimated two to three percent of all live births are post-term. In Europe, the rate varies from 0.4 percent in Austria to 8.1 percent in Denmark.

The study is published in the journal Paediatric and Perinatal Epidemiology.