Firstborn women are more likely to be overweight or obese than their sisters, a New Zealand-led study with Swedish scientists revealed Friday.
The study led by Auckland University's Liggins Institute, the largest of its kind among women, found that firstborns were almost 30 percent more likely to be overweight, and 40 percent more likely to be obese than their second-born sisters.
Professor Wayne Cutfield, of the Liggins Institute, said the study of 13,406 pairs of adult Swedish sisters backed up the findings of earlier studies by his team in firstborn adult men and in children of both sexes.
"Collectively, these studies show that both men and women who are born first are at greater risk of being overweight or obese," Cutfield said in a statement.
The earlier studies also found firstborn men and firstborn children in general were more insulin resistant -- a risk factor for type 2 diabetes -- and the children had higher blood pressures, which was a risk factor for hypertension.
However, firstborns should not treat the findings as a prediction that they would become obese, diabetic or hypertensive.
"The differences of about 20 to 25 percent in obesity and insulin sensitivity between firstborns and those born later are not large enough to be a major determining factor. What this information about health risks does is to empower firstborns so they can make positive choices about diet and exercise," he said.
Researchers had yet to find the reason for the differences between firstborns and those born later, but Cutfield suggested it was 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," he said.
The findings could also add to understanding of the obesity epidemic.
"The steady decrease in family size over the last century has created a higher proportion of firstborns. That may be a contributing factor to the steady increase we are seeing in the adult body mass index or BMI around the globe," he said.
