The Unexpected Way Birth Order Affects Your Weight, According To Science

Pay special attention, firstborns.

sisters hugging Getty

If you're like many women in the U.S., your parents, grandparents, and/or overly-opinionated aunts and uncles may compare you to your sister(s) at pretty much every family gathering you attend.

In particular, your bodies may be constantly picked apart because your shapes and sizes are so different.

If you're the eldest sister among your siblings and feel constantly judged again your little sister — who's shaped like Kendall Jenner, while your figure is more Kim Kardashian design — chances are, you're not alone, as research now shows there's a scientific reason your younger sister weighs less than you.


How does birth order impact weight and size differences between sisters both as children and adults?

In a study of over 26,000 Swedish women published in The Journal of Epidemiology & Community Health, researchers found that "firstborns were nearly 30 percent more likely to be overweight and 40 percent more likely to be obese than their younger siblings were."

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These findings appear to back up similar studies of brothers, which found that firstborn males are "not only more likely to be heavier, but also more likely to develop high blood pressure and insulin resistance than were their younger brothers."


Further, the study's authors suggest these findings may provide an explanation for rising obesity rates around the world.

As fewer couples choose to have more than one child, they posit, the ratio of kids born with this 30 to 40 percent likelihood of being overweight increases throughout the general population.

While this may sound as though firstborn children are destined to be fat, the new research isn't meant to be judgmental or shaming. Rather, it provides a possible answer as to why you and your sister have such different body types, and why it seems impossible for either of you to achieve your desired, opposite body type (as the grass often appears to be greener between sisters).

Gaining this knowledge that your differences in weight and size have nothing to do with either of you being better or worse, or more or less disciplined, than the other can hopefully be used as a springboard from which to calm any lingering sibling rivalry.


Interestingly enough, this study doesn't cite genetics as the suspected cause for these differences in weight among siblings.

Instead, the reason your birth order affects your future weight is all about your mother's uterus.

"During a woman’s first pregnancy, blood vessels to the placenta are narrow," the researchers say. “And this information has led to the hypothesis that firstborns were exposed to in utero compromise, which reprograms metabolism and the regulation of fat.”

This results in firstborns having a slower metabolism than their younger siblings.

This research suggests that even from the womb, the environment plays a key role in determining whether or not someone will be overweight.


So on a positive note, you now have a possible explanation as to why you and your siblings are shaped so differently, as well as some extra motivation to find a diet or exercise program that targets your specific metabolic needs.

Plus, the next time someone says, "Why aren't you skinny like your little sister?" you can make them feel awkward by responding, "Because of my mom."

Because as Freud always (basically) said, if it's not one thing, it's your mother ...


RELATED: Being Overweight Actually Means You Live Longer, Says Science

Stacy Narine is a regular contributor to YourTango.