title: High birth weight and perinatal mortality among siblings: A register based study in Norway, 1967-2011.
abstract: BACKGROUND
  Perinatal mortality according to birth weight has an inverse J-pattern. Our aim was to estimate the influence of familial factors on this pattern, applying a cohort sibling design. We focused on excess mortality among macrosomic infants (>2 SD above the mean) and hypothesized that the birth weight-mortality association could be explained by confounding shared family factors. We also estimated how the participant's deviation from mean sibling birth weight influenced the association.
  
  
  RESULTS
  We included 1 925 929 singletons, born term or post-term to mothers with more than one delivery 1967-2011 registered in the Medical Birth Registry of Norway. We examined z-score birth weight and perinatal mortality in random-effects and sibling fixed-effects logistic regression models including measured confounders (e.g. maternal diabetes) as well as unmeasured shared family confounders (through fixed effects models). Birth weight-specific mortality showed an inverse J-pattern, being lowest (2.0 per 1000) at reference weight (z-score +1 to +2) and increasing for higher weights. Mortality in the highest weight category was 15-fold higher than reference. This pattern changed little in multivariable models. Deviance from mean sibling birth weight modified the mortality pattern across the birth weight spectrum: small and medium-sized infants had increased mortality when being smaller than their siblings, and large-sized infants had an increased risk when outweighing their siblings. Maternal diabetes and birth weight acted in a synergistic fashion with mortality among macrosomic infants in diabetic pregnancies in excess of what would be expected for additive effects.
  
  
  CONCLUSIONS
  The inverse J-pattern between birth weight and mortality is not explained by measured confounders or unmeasured shared family factors. Infants are at particularly high mortality risk when their birth weight deviates substantially from their siblings. Sensitivity analysis suggests that characteristics related to maternal diabetes could be important in explaining the increased mortality among macrosomic infants.
authors.affiliation: Department of Occupational Medicine and Epidemiology, National Institute of Occupational Health, Oslo, Norway.|Department of Community Medicine, Institute of Health and Society, University of Oslo, Oslo, Norway.
authors.forename: Petter
authors.identifier: http://orcid.org/0000-0003-0469-5033
authors.initials: P
authors.lastname: Kristensen
authors.affiliation: Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY, United States of America.
authors.forename: Katherine M
authors.initials: KM
authors.lastname: Keyes
authors.affiliation: Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY, United States of America.|New York State Psychiatric Institute, New York, NY, United States of America.
authors.forename: Ezra
authors.initials: E
authors.lastname: Susser
authors.affiliation: Department of Occupational Medicine and Epidemiology, National Institute of Occupational Health, Oslo, Norway.
authors.forename: Karina
authors.initials: K
authors.lastname: Corbett
authors.affiliation: Department of Occupational Medicine and Epidemiology, National Institute of Occupational Health, Oslo, Norway.
authors.forename: Ingrid Sivesind
authors.identifier: http://orcid.org/0000-0003-0830-284X
authors.initials: IS
authors.lastname: Mehlum
authors.affiliation: Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway.|Medical Birth Registry of Norway, Norwegian Institute of Public Health, Bergen, Norway.
authors.forename: Lorentz M
authors.initials: LM
authors.lastname: Irgens
mesh_terms: D001724:Birth Weight / Q000502:physiology*; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007232:Infant, Newborn, Diseases / Q000453:epidemiology / Q000401:mortality; D008297:Male; D009035:Mothers; D009664:Norway; D066087:Perinatal Death; D054238:Perinatal Mortality; D011247:Pregnancy; D035781:Siblings