Attention, big sisters: First-born women are more likely to be overweight or obese than their younger sisters, a new study suggests. The Washington Post reports that researchers at the University of Auckland looked at 13,406 pairs of Swedish sisters and found the older sibling was 29% more likely to be overweight and 40% more likely to be obese. First-born sisters also had a BMI 2.4% higher than their sisters in the study, which looked at the women during their own pregnancies, according to CBS News. Interestingly, the Post points out first-born women tended to be lighter at birth than their younger sisters. This was the largest study ever on the effects of birth order and also the first study to look at pairs of siblings to account for genetic and home-life factors.
The results from the study fit with those from earlier studies that showed first-born men have a higher BMI at middle age and are more likely to suffer from diabetes and high blood pressure. Study author Wayne Cutfield tells CBS first-borns may be fatter because they received less nutrients through the placenta than their siblings, preprogramming their body to store more fat after they're born. In subsequent births, babies get more nutrients because mom's blood vessels are wider. Another researcher has a simpler explanation: First-borns tend to get overfed. The study, published in the Journal of Epidemiology & Community Health, also claims this may be why worldwide average BMIs are on the rise; increasingly smaller family sizes mean more first-borns skewing the average. (But at least first-borns have higher IQs, right?)
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