First-born women are 40 per cent more likely to be obese than their younger sisters, according a study led by a Kiwi academic.
Professor Wayne Cutfield of the University of Auckland's Liggins Institute said the research 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."
The study found first-borns were nearly 30 per cent more likely to be overweight, and 40 per cent more likely to be obese than their second-born sisters.
It also found mounting evidence that first-borns had an increased risk of health problems such as type 1 diabetes and high blood pressure later in life.
The report was published online today in the Journal of Epidemiology & Community Health.
Professor Cutfield said the research supported findings of studies of first-born adult men and of children. But there was no guarantee that first-borns would be overweight or obese.
"What this information about health risks does is empower first-borns so they can make positive choices about diet and exercise," he said.
The reason from the differences remain unknown, but it could be down to differences in blood supply to the placenta between first and subsequent pregnancies, Professor Cutfield said.
"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 first-born is at risk of storing more fat and having insulin that works less effectively."
The study examined data collected at the first antenatal visit of Swedish women aged over 18, between 1991 and 2009.
- Susan Strongman, The New Zealand Herald