1. First-born sisters were found to have to have a greater body mass index (BMI) than their second-born sisters.
2. First-born sisters were 30% more likely to be overweight and 40% more likely to be obese than their second-born sisters.
Evidence Rating Level: 3 (Average)
Study Rundown: It is not known exactly what which phenotypic differences are more likely between children who are first-born and those who are second-born or third-born. Studies indicate that first-born men may be taller, heavier, and of greater BMI than their younger siblings, who show a progressive reduction in height and weight. This study was conducted to assess whether birth order was associated with height and BMI in women, as previous studies mostly studied the effects of birth order in men. In analyzing over 13,000 Swedish pairs of sisters, first-born women were found to have a BMI 0.57 kg/m2 greater than in their younger sister at an early stage in pregnancy. Further, first-born women had a 30% greater risk of being overweight a 40% increased risk of being obese than their younger sister. While first-born sisters were slightly taller, the difference was negligible.
This study benefits from being one of the largest of its kind to study birth order effects on women. By utilizing only sibling pairs, the authors were able to control for genetic factors and certain environmental factors as well. However, it is reasonable to expect that with age, effects of environmental factors would diverge for sisters, but the authors did not control for dietary intake, physical activity levels, or socioeconomic background. Overall, the study is in line with the larger studies on men, suggesting that first-born women may have an increased risk of having a higher BMI and of being overweight or obese compared to their second-born sisters, though there are study limitations preventing us from making this generalization.
In-Depth [retrospective cohort]: This study looked at data from 13,406 pairs of Swedish sisters, 26,812 women in total, to determine whether being the first or second born female impacted BMI. The authors conducted the analysis by collecting information from antenatal visits during women’s pregnancies, the majority before 15 weeks of gestation, between 1991 and 2009. Early in pregnancy, first-born women had a BMI that was BMI 0.57 kg/m2 greater than their younger sister (p<0.0001). The first-born sister had a 29% increased risk of being overweight [OR 1.29 (CI95% 1.21-1.38)] and a 40% increased risk of being obese [OR 1.40 (CI95% 1.26-1.54)] compared to their second-born sister. While the first-born was 1.2mm taller than their second-born sister, this difference was marginal (p=0.026).
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