A study led by a Kiwi academic has found that first-born women are 40 per cent more likely to be obese than their younger sisters.
And with single child families on the rise, Professor Wayne Cutfield of the University of Auckland's Liggins Institute, said the study he led could help explain why weight gain and obesity are becoming more common worldwide.
"The steady decrease in family size over the last century has created a higher proportion of firstborns," he said.
"That may be a contributing factor to the steady increase we are seeing in the adult body mass index or BMI around the globe."
Professor Cutfield said the study found firstborns were nearly 30 per cent more likely to be overweight, and 40 per cent more likely to be obese than their second-born sisters.
The study also found there was mounting evidence that firstborns had an increased risk of adverse health outcomes later in life - like type 1 diabetes and high blood pressure.
The report (entitled 'Firstborns have greater BMI and are more likely to be overweight or obese: a study of sibling pairs among 26,812 Swedish women) was published online today in the Journal of Epidemiology & Community Health.
Professor Cutfield said the study backed up findings of other studies by his team of firstborn adult men and in children.
"Collectively, these studies show that both men and women who are born first are at greater risk of being overweight or obese," he said.
But he said it was important for people to understand that being first born did not mean all oldest children would be overweight or obese.
"What this information about health risks does is empower firstborns so they can make positive choices about diet and exercise," he said.
Researchers have yet to find the reason for the differences between firstborns and those born later, but Professor Cutfield proposes that it is due to differences in the blood supply to the placenta.
"In a first pregnancy, the blood vessels to the placenta are narrower. This reduces the nutrient supply, thus reprogramming the regulation of fat and glucose, so that in later life the firstborn is at risk of storing more fat and having insulin that works less effectively," Professor Cutfield said.
The study examined data collected at the first antenatal visit, from Swedish women aged over 18 years, between 1991 and 2009.