By Stacey Leasca
Sisters Laughing Together
Credit: JGI/Tom Grill/Getty Images

Let's face it; older siblings kind of get the short end of the stick. They are the tester kid for parents, the ones who seem to get in trouble for everything their younger siblings don't, and it turns out, they may be genetically predisposed to weigh more than their younger siblings, too.

In 2015, researchers from New Zealand and Sweden teamed up to dig into data from the Swedish Birth Register. The team analyzed data dating back to 1973 and focused on the data showing birth weights in women.

The authors then honed in on the period between 1991 and 2009, CBS News explained, and looked at women who were at least 18 at the time of their first pregnancy and whose mother was at least 18 years old at the time of her birth. In total, the researchers tracked the data on just over 13,000 pairs of Swedish sisters.

After combing through the extensive data, the team found that firstborn women are 29 percent more likely to be overweight than their younger sisters, and 40 percent more likely to be obese. Moreover, the team found that the older sisters are just a bit taller as adults, and had higher BMIs when they were pregnant.

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But, all that also adds up to more bad news for the elder sisters in the room. As the researchers noted in their findings, which were published in the Journal of Epidemiology & Community Health, firstborns also have a higher risk of diabetes, high blood pressure, and other diseases as well.

How could this be? Though the study's lead author, Professor Wayne Cutfield from the Liggins Institute at the University of Auckland, didn't want to draw a definitive conclusion, he did hypothesize that there may be a change in the blood supply to the placenta between first and second pregnancies. As he explained to CBS, blood vessels may be more narrow in the first pregnancy, causing a reduction in the nutrient supply, and thus causing the first child to have more difficulty with insulin regulation.

Of course, there are other theories too. As Dr. Maria Peña, Director of the Center for Weight Management at Lenox Hill Hospital in New York City, told CBS, it may simply come down to nurture over nature.

"In many cultures, moms are more meticulous with their firstborns," she said. "With the very first born, everyone's helping out and over-feeding the baby, making sure it's at a 'healthy weight.' But with second children, parents know what to expect and they're not so overprotective so maybe they feed them a little less."

So, at the very least, older sisters can walk away from this story knowing they were the favorite child.