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Firstborns had 29 percent greater odds of being overweight and 40 percent greater odds of being obese than sisters born second, the researchers said. In addition, firstborns were also slightly taller.
"This is the fourth study we have done to characterize the health risks of firstborn in four different populations," said lead researcher Dr. Wayne Cutfield, a professor of pediatric endocrinology at the Liggins Institute of the University of Auckland in New Zealand.
"If you look at the health risks of those that are firstborn, you find that firstborns are more insulin resistant than later borns, which is a risk factor for diabetes, and they have higher blood pressure than later borns," he said.
However, the new study is an observational study, so no definitive conclusions can be drawn about cause-and-effect between birth order and later weight status, Cutfield pointed out.
Still, Cutfield thinks that these risks might be due to less of the mother's blood reaching the placenta during her first pregnancy than in later pregnancies because blood vessels are narrower in the first pregnancy.
In addition, firstborns tend to weigh less than later borns, which might be a result of less nutrients reaching the first fetus, he said.
As families have been getting smaller, firstborns make up a larger part of the population, which might explain some of the obesity epidemic, he suggested. "It's a small contributor. It's not a major contributor," according to Cutfield.
He said that knowing the health risk of being firstborn is helpful because it empowers firstborns to make decisions about their lifestyle. Living a healthy lifestyle can reduce the risk of obesity, high blood pressure and diabetes, he pointed out.
"There are minor health risks associated with being firstborn," Cutfield said. "I don't want firstborns to think they will become obese or get diabetes or high blood pressure -- it is a risk factor, and the risk of getting a disease is a combination of risk factors, not just a single risk factor," he said.
The report was published online Aug. 26 in the Journal of Epidemiology & Community Health.
Using the Swedish Birth Register, Cutfield's team collected data on girls born between 1973 and 1988 who were pregnant between 1991 and 2009. Complete data were available for more than 13,000 sister pairs -- just under 27,000 women in total.
The researchers noted that this study's findings corroborate previous studies that have shown a link between greater weight and height in firstborns in men.
Dr. David Katz, president of the American College of Lifestyle Medicine, said, "This study establishes an association in females, already seen in males, between the likelihood of obesity in adulthood and place in the birth order, but by design, it cannot say for sure why such an association exists."
Perhaps firstborn children are less well-nourished in the womb or excessively nourished in early childhood. Perhaps there are life stresses unique to being the firstborn child that somehow pertain, he suggested.
Some factors related to obesity are not a matter of individual choice -- no one chooses their place in a birth order. This study makes us consider aspects of obesity that are not related to personal responsibility, Katz said.
Being born first may play a small role in obesity, but the obesity epidemic is largely a consequence of society's obsession with unhealthy food choices and a sedentary lifestyle, Katz said.
"Birth order is not a modifiable risk factor, but the obesogenic environment and a lifestyle at odds with weight control and health promotion certainly are," he said. "Our attention, as ever, should be directed there."
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