While firstborns may bask in the glory of having slightly higher IQ than their younger siblings, when it comes to their body weight, the news is not so good. A recent study revealed that firstborn women are more likely to be overweight.
The study looked at sister pairs from the Swedish national register
In line with previous research that birth order affects body weight in men, scientists from the University of Auckland wanted to find out if a similar relationship existed for sisters too. Published in the Journal of Epidemiology and Community Health, this study examined the prenatal clinic records from the Swedish National Birth Register and followed up for a period of 20 years spanning 1991 to 2009. The register was started in 1973 and has the record of virtually all births in Sweden. Totally, 303,301 girls were born between 1973 and 1988 and these women gave birth between 1991 and 2009. Only first and second born women were selected for the study resulting in 13,406 sister pairs. Their weight and height were assessed at birth and, next, at the time of their first prenatal visit (3 months into pregnancy). Twins were excluded from the study.
Firstborn women: Birth order influences their bodyweight
Firstborn women were found to be lighter at birth than their younger sisters. However, during their first three months of pregnancy, the firstborn sisters had a 2.4% higher BMI. Overall, firstborns were 29% more likely than their second born sisters to be overweight and 40% more likely to be obese. In addition, firstborn women were more likely to be taller (1.2 mm) than their younger siblings in adulthood. Though this might seem like a negligible difference, it corroborates what researchers call the resource dilution hypothesis, which states that as the size of a family increases, there’s less to go around, thereby diminishing genetic resources for each additional child.
The reduction in family size may be contributing to the rise in adult BMI
This study is purely observational and even though comparing sisters ruled out common genetic factors, nutritional and physical activity variables could have been responsible for the observed difference. Still, the study examined a large sample of the population, and found a considerable spike in the BMI of firstborn women. Also, this supports similar findings in men, where birth order affects both their height and weight. This warrants further studies to explore the metabolic health issues that are unique to firstborns.
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Firstborn Women More Likely to Be Overweight
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