The most recent research has to do with weight, and, in this regard, it turns out that being born first may not be so good after all.
At birth, those born first were more likely to be slightly lighter than their sisters according to the analysis. But something changed as they grew up. As adults, first-borns were significantly more likely to have higher BMI and be overweight or obese.
Specifically, first-borns were 29 percent more likely to be overweight and 40 percent more likely to be obese than their second-born sisters.
The findings are consistent with previous research that showed that birth order may be inversely associated with BMI. A New Zealand study on 50 middle-aged men who are overweight, for instance, found that first-borns had a BMI that is 1.6 kg per square meters greater than second-borns.
Wayne Cutfield, a researcher at the Liggins Institute at the University of Auckland and the lead author of the women's study, wrote in the study that the triggers and mechanisms for the findings are unknown. But, according to CBS, he hypothesized that it may have to do with what happened to the women in utero.
He explained that there may be a shift in the blood supply to the placenta that occurs between the first and later pregnancies. In the first pregnancy, the blood vessels may be narrower, which reduces the nutrient supply and may make them store more fat.
"[O]urs was not only the largest study of birth order effects on women, but was also the only one focusing specifically on sibling pairs, to largely account for genetic factors and the early life environment," Cutfield and his co-investigators wrote.
The study is part of mounting evidence that shows that first-borns may be at an increased risk of adverse health outcomes, including diabetes type 1 and hypertension later in lives.