title: First-borns have greater BMI and are more likely to be overweight or obese: a study of sibling pairs among 26,812 Swedish women.
abstract: BACKGROUND
  A number of large studies have shown phenotypic differences between first-borns and later-borns among adult men. In this study, we aimed to assess whether birth order was associated with height and BMI in a large cohort of Swedish women.
  
  
  METHODS
  Information was obtained from antenatal clinic records from the Swedish National Birth Register over 20 years (1991-2009). Maternal anthropometric data early in pregnancy (at approximately 10-12 weeks of gestation) were analysed on 13,406 pairs of sisters who were either first-born or second-born (n=26,812).
  
  
  RESULTS
  Early in pregnancy, first-born women were of BMI that was 0.57 kg/m(2) (2.4%) greater than their second-born sisters (p<0.0001). In addition, first-borns had greater odds of being overweight (OR 1.29; p<0.0001) or obese (OR 1.40; p<0.0001) than second-borns. First-borns were also negligibly taller (+1.2 mm) than their second-born sisters. Of note, there was a considerable increase in BMI over the 18-year period covered by this study, with an increment of 0.11 kg/m(2) per year (p<0.0001).
  
  
  CONCLUSIONS
  Our study corroborates other large studies on men, and the steady reduction in family size may contribute to the observed increase in adult BMI worldwide.
authors: Liggins Institute, University of Auckland, Auckland, New Zealand.
authors: José G B
authors: JG
authors: Derraik
authors: Department of Women's and Children's Health, Uppsala University, Uppsala, Sweden.
authors: Fredrik
authors: F
authors: Ahlsson
authors: Department of Women's and Children's Health, Uppsala University, Uppsala, Sweden.
authors: Maria
authors: M
authors: Lundgren
authors: Department of Women's and Children's Health, Uppsala University, Uppsala, Sweden.
authors: Björn
authors: B
authors: Jonsson
authors: Liggins Institute, University of Auckland, Auckland, New Zealand.
authors: Wayne S
authors: WS
authors: Cutfield
mesh_terms: D001722:Birth Order; D015992:Body Mass Index; D005260:Female; D006801:Humans; D016013:Likelihood Functions; D008297:Male; D009765:Obesity / Q000453:epidemiology*; D012042:Registries; D012307:Risk Factors; D035781:Siblings; D013548:Sweden / Q000453:epidemiology
keywords: Epidemiology of chronic diseases; GENDER; OBESITY