Are Firstborn Sisters More Likely To Be Overweight?

Birth order trends seem to always interest readers, and a recent look at firstborn sisters is no less intriguing. A study of firstborn sisters from the University of Auckland in New Zealand examined the patterns of weight among firstborn sisters. Firstborn brothers have already shown, in previous studies, to be both taller and at greater risk of obesity than their younger brothers, but until now, no one knew if that same trend could be seen among firstborn sisters.

As it turns out, like firstborn brothers, firstborn sisters are also more likely to be overweight, according to the study which was published in the Journal of Epidemiology & Community Health.

“This is important as there is well-described sexual dimorphism in association with early life events, with contrasting effects on long-term health and disease observed in men and women,” the authors wrote, suggesting that firstborn sisters are at 40 percent greater risk of weight problems than their secondborn sisters. The researchers can’t say if this trend in firstborn sister can be seen across the globe, but it is the case in Sweden at least. The New Zealand researchers used the Swedish Birth Register as their data set. It included over 300,000 women born between 1973 and 1988.

When the data was examined, the researchers identified 13,406 sets of sisters. Their height and weight at birth were looked at, as well as their information years down the line as the sisters had their first prenatal visits. Interestingly, at birth, firstborn sisters weighed less, but during the first trimester of their pregnancies, firstborn women averaged a body mass index that was 2.4 percent higher than their secondborn sisters. Additionally, firstborn sisters were 20 percent more likely to be overweight and 40 percent more likely to be clinically obese than their secondborn sisters.

Just like the males, firstborn sisters were also taller adults, but the difference was barely measurable. The average height advantage of firstborn sisters during adulthood was only 1.2 millimeters. Interestingly, the more siblings born before women in birth order, the shorter the women were as adults. The researchers suspect this might be a case of the “resource dilution hypothesis.” This concept, according to Medical News Today, is the idea that parents resources might be spread more thinly as they have more kids. As far as why firstborn sisters might be more prone to obesity than secondborn sisters, they suspect that firstborn babies might be more likely to have nutritional deficiencies, perhaps because their mothers were less-educated or prepared during their first pregnancies.

Right now, it all comes down to statistics, but further research into these findings about firstborn sisters will examine if their actual metabolisms might be different than secondborn sisters.

[Photo via Pixabay]