First-born women may be more likely to be overweight or obese than their younger sisters, according to a new study. But that's not the only way being a big sister might affect your health.
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Struggling with your weight while your baby sister is thin as a rail? Turns out, you're probably not alone. First-born women may be more likely to be overweight or obese than their younger sisters, according to a new study in the Journal of Epidemiology & Community Health.

For the study, researchers looked at more than 13,000 pairs of Swedish sisters and found that the older siblings were 29% more likely to be overweight and 40% more likely to be obese by their mid-twenties than their younger counterparts. (Keep in mind that this isn't a huge jump in risk; that means older siblings have 1.29 times the risk of being overweight and 1.4 times the risk of being obese than young sibs, rather than say 10 or 20 times the risk.)

If it's proven, it is unclear why older sisters may end up heavier, but one hypothesis is that during a first pregnancy, the blood vessels in a mother's uterus are more narrow. "And this information has led to the hypothesis that first-borns were exposed to in utero compromise, which reprograms metabolism and the regulation of fat," study co-author Wayne Cutfield explained to In other words, a lower energy supply in the womb may lead to a bigger appetite and the way the body regulates weight later on. Or it's possible that the care and feeding of a first-born differs from siblings in a way that impacts weight later on.

Interestingly, this is not the first study to link health issues to being the oldest. For one thing, this study echoes previous findings that first-born men are also more likely to be heavier than their kid brothers.

But also, eldest children may be more likely to have reduced insulin sensitivity (a problem linked to the development of type 2 diabetes) and higher blood pressure compared to later-born children, a 2013 study in the Journal of Clincial Endocrinology & Metabolism found. This may set the stage for a higher likelihood of metabolic and cardiovascular diseases in adulthood, the authors concluded.

Another possible health struggle: allergies. In 2011, Japanese researchers surveyed the parents of more than 13,000 children from age 7 to 15 and found that hay fever and food allergies were more common among eldest children.

What about personality?

While smaller studies have linked being the oldest to having more smarts, better language skills, and even more sexual partners, forthcoming research from the Journal of Research in Personality suggests all that is mostly bunk. The researchers looked at the connection between birth order and 19 different personality traits and intelligence attributes (like verbal or math ability) in 377,000 U.S. high school students only to conclude that when you account for factors like age, sex, socio-economic status, and family structure, it is very unlikely that birth order means very much at all.

It's sort of a bummer, but the next time your sibling spouts on about the oldest being the smartest or the youngest being more rebellious, here's your ammunition.