Great, now there's more to squabble about with your siblings: A new study, published in the Journal of Epidemiology & Community Health, shows that older sisters are at a disadvantage when it comes to weight gain.

Researchers from New Zealand and Sweden studied medical records of 13,000 pairs of Swedish sisters who were either first born or second born, and tracked them over 20 years. This was the largest study of its kind among women, and it backs up similar results when men were studied.

They found that firstborn women are 29% more likely to be overweight, and 40% more likely to be obese, than their younger sisters. They were also slightly bigger at birth, ever so slightly taller as adults, and had higher BMIs when they were pregnant. And the news keeps getting worse for first-borns: According to the study, previous research has found that firstborns have a higher risk of diseases like diabetes and high blood pressure.

The researchers say that since this is an observational study, there's no way to prove exactly why this happens. According to CBS News, some scientists say it might have to do with nutrition levels in the uterus or placenta, the organs that reprogram how fat and glucose are stored in the body. Others point to parents who coddle first-borns and potentially over-feed them. The study argues that the obesity epidemic might be stemming from parents choosing to have fewer children.

So if you're the first-born sister, there's not much you can do about its effect on your weight. But if you're the younger sibling, you now have a new reason to gloat about it during Thanksgiving.

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