Firstborn girls more likely to be overweight; at greater risk of obesity

Birth order has been said to have profound and lasting effects on a person's psychological development, but a new study claims that it's more than just that. In fact, the study found that big sisters may literally become big once they are adults. Health reports that the study, which was published in the Journal of Epidemiology and Community Health, revealed that firstborn girls are 29 percent more at risk for being overweight and 40 percent more likely of being obese compared to their younger sisters. Additionally, the study found that firstborn children are slightly taller than their younger siblings. Dispatch Times reports that this is the largest study done exclusively in adult female pairs, and study authors said that they analyzed for different populations for the research.

Dr. Wayne Cutfield, lead researcher and a professor of pediatric endocrinology at the Liggins Institute of the University of Auckland in New Zealand said: "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."

According to the study authors, there is a 20 to 25 percent difference in obesity and insulin resistance between firstborns and their succeeding siblings. Cutfield explained that changes in blood supply to the placenta when the woman is pregnant may cause the difference.

"This reduces nutrient supply, thus reprogramming the regulation of fat and glucose, so that in later life, the individual is at risk of storing more fat and having insulin that works less effectively," said Cutfield.

He added: "The steady reduction in family size may be a contributing factor to the observed increase in adult BMI (body mass index) worldwide, not only among men, but also among women."

However, Cutfield explained that being firstborn is linked to minor health risks. "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. 

Dr. David Katz, president of the American College of Lifestyle Medicine also commented: "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."

Cutfield concluded: "What this information about health risks does is empower firstborns so they can make positive choices about diet and exercise."