The study found older sisters are 29 percent more likely to be overweight and 40 percent more likely to be obese than their next-younger sister.
Bad news, big sisters: A new study finds that firstborn girls are more likely to be overweight or obese than their second-born sisters.
The findings are based on data collected from more than 13,400 pairs of sisters born in Sweden.
According to the study, older sisters are 29 percent more likely to be overweight and 40 percent more likely to be obese than their next-younger sister. Firstborn girls are also, on average, a little taller than their sisters as adults.
The results were published last week in the Journal of Epidemiology & Community Health.
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The researchers say the weight discrepancy does not begin at birth. Indeed, firstborn girls were usually a bit smaller as infants than their sisters, by an average of about 3 ounces.
The women who participated in the study were at least 18 years old and pregnant. Their height, weight and body mass index were measured as part of their first prenatal doctor visit, usually at around 10 to 12 weeks of gestation.
The researchers were able to determine how big the women were at birth by consulting the Swedish Birth Register, which was started in 1973 and contains records on more than 99 percent of all births in Sweden.
The authors note that the study population is relatively homogeneous, and so the results may not be applicable to people of other ethnic groups or populations with bigger weight problems.
However, they write that their findings support the results of smaller studies in Italy and Poland that detail similar trends. An earlier study that looked at 1 million Swedish men also found that firstborns were taller, heavier and had a greater body mass index than their younger siblings.
The big question is why.
Unfortunately, this study does not provide an answer.
The authors do cite one hypothesis in the paper that was first proposed in 2013: Firstborn children suffer a degree of undernutrition in utero compared to their siblings, which could lead to a greater chance of health problems down the road.
“There is mounting evidence that firstborns have an increased risk of adverse health outcomes later in life,” they write.
Sometimes first isn’t best.