Link Between Birth Order and Weight- Research

What does birth order have to do with weight? A lot, according to a new study. 

Simply being the firstborn sister may put a woman at higher risk of becoming overweight or obese down the road, new research published in the Journal of Epidemiology & Community Health suggests.

In the study, conducted at the University of Auckland’s Liggins Institute, Australian and Swedish researchers evaluated data on 13,406 pairs of sisters — not including twins — born between 1991 and 2009 from the Swedish Birth Register.

Scientists found that firstborn sisters were 29 percent more likely to be overweight later in life than their second-born siblings. They were also 40 percent more likely to be obese despite being just a little bit taller than their younger sisters. To remove other influences that may affect weight, such as environment and genetics, researchers looked specifically at sisters in determining whether firstborn status may contribute to weight issues later in life.

This new research seems to coincide with other observational studies conducted by Liggins Institute researchers of men and children that also showed a link between birth order and weight. These studies dug even deeper into the data on firstborns, which indicated being oldest was also associated with higher blood pressure and insulin resistance.

Why does being born first mean getting the shorter end of the stick, health-wise? Although no research has confirmed a cause, lead study author Wayne Cutfield, a professor at the Liggins Institute, tells Yahoo Health it may have to do with a difference in blood supply to the placenta during the first pregnancy compared with later pregnancies.

The vessels are narrower during the initial pregnancy, says Cutfield via email. “This reduces the 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,” he writes.

Cutfield says being the oldest sibling is probably just a “small contributor” to the overall odds of becoming obese or overweight in later years. From his research, the differences in both obesity and insulin resistance between firstborns and later-borns is somewhere between 20 and 25 percent — a variation that is not great enough to make it be considered a major risk factor.

However, knowing your disease risk should always motivate you to make better decisions about your health, Cutfield says. Just as you might try harder to eat well-balanced meals and exercise regularly if you have a family history of heart disease or diabetes, with more research on firstborn health risks, this group could be similarly empowered to take charge of their well-being.

Jenna Birch