The trend towards having smaller families could be helping drive the worldwide obesity epidemic, new research suggests.
Scientists in New Zealand and Sweden have found firstborn women are 30 percent more likely to be overweight than their younger sisters, and 40 percent more likely to be obese.
"The steady decrease in family size over the last century has created a higher proportion of firstborns," explains senior investigator Professor Wayne Cutfield of the Liggins Institute at the University of Auckland.
"That may be a contributing factor to the steady increase we are seeing in the adult body mass index or BMI around the globe."
BMI measures the ratio between a person's height and weight. While it can be inaccurate when applied to an individual – for example, All Black captain Richie McCaw has a BMI of 30, making him borderline obese – it is designed to measure entire populations.
The researchers looked at more than 13,000 pairs of adult Swedish sisters, and the findings back up previous studies that focused on men.
"Collectively, these studies show that both men and women who are born first are at greater risk of being overweight or obese," says Prof Cutfield.
Just why isn't yet known, but Prof Cutfield says it could have something to do with differences in antenatal blood supply from the mother 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."
And with science effectively proving firstborns more likely to be overweight, Prof Cutfield says this could partly explain the obesity epidemic sweeping the Western world.
But distraught firstborns shouldn't treat the findings as a reason to give up hope, he says.
"What this information about health risks does is to empower firstborns so they can make positive choices about diet and exercise."