Girl babies born more than a week after they're due could be 12 per cent more likely to grow up obese, according to new research.

A study by Auckland University's Liggins Institute found women born very post-term, at 43 weeks or more, were on average 1kg heavier and had a greater body mass index (BMI) than women born at 38 to 40 weeks.

Researchers from the Liggins Institute 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," Liggins senior research fellow Dr José Derraik said.


"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.

"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 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," Dr Derraik said.

"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."

Dr Derraik hoped the findings would empower women born post-term to make better lifestyle choices to minimise their risks of developing diabetes or heart disease.

In New Zealand, an estimated two to three per cent of all live births are post-term.

In Europe, the rate varies from 0.4 per cent in Austria to 8.1 per cent in Denmark.