Bad news for first born sisters…
Older sisters are more likely to be overweight and obese when compared to their younger siblings. According to a large study performed on 13, 406 Swedish sister pairs. The results corroborate with previous studies that found older brothers to be both, heavier, and taller, than their younger siblings. The study showed that at birth, firstborns weighed a little less than the second born sisters, however their BMI was around 2.4% higher during their first 3 months of pregnancy.
Overall it was found that firstborn women were 29% more likely to be overweight, and 40% more likely to be obese during their mid-20s, furthermore they were an average of 1.2mm taller. Sisters with a high number of siblings were also more likely to be shorter and the researchers hypothesised that this could be due to a phenomenon known as ‘resource dilution’ – the idea that parental resources are spread more thinly across a higher number of children.
It was unclear why older sisters seemed to be heavier, however the researchers hypothesised that changes in the womb after the first pregnancy could be a potential cause. It is possible that during the first pregnancy, blood vessels are narrower in the placenta, limiting flow. The body then adapts to this to store more fat and glucose. For subsequent pregnancies the blood flow is altered to stop this from happening. Although the findings confirm the results in other studies involving men, more research is needed to see whether there is an associated increase in metabolic risk in first born women.
