title: Birth weight and risk of asthma in 3-9-year-old twins: exploring the fetal origins hypothesis.
abstract: OBJECTIVE
  To examine the relationship between birth weight and risk of asthma in a population of twins.
  
  
  METHODS
  Birth weight of all live twins (8280 pairs) born in Denmark between 1994 and 2000 was linked to information on asthma obtained from parent-completed questionnaires at age 3-9 years. Conditional logistic regression was used to calculate the risk of asthma.
  
  
  RESULTS
  Subjects with a history of asthma at age 3-9 years weighed on average 122 g (95% CI 85 to 160) less at birth than subjects who had not developed asthma, p<0.001. There was a linear increase in asthma risk with decreasing birth weight, OR (per 100 g) 1.04 (95% CI 1.03 to 1.05), p<0.001. Within twin pairs, the lower birthweight twin had a significantly increased risk of asthma compared with the heavier co-twin (11.3% vs 9.9%), OR 1.30 (95% CI 1.10 to 1.54), p=0.002. The result remained significant after adjusting for sex, birth length and Apgar score, OR 1.31 (95% CI 1.03 to 1.65), p=0.027. The risk tended to be higher in monozygotic co-twins compared with dizygotic co-twins, especially for high birth weight differences.
  
  
  CONCLUSIONS
  Low birth weight is a risk factor for asthma independently of gestational age, sex, birth length and Apgar score, but this may be due, in part, to residual non-genetic confounding factors. This finding lends support to the "fetal origins hypothesis" suggesting undisclosed prenatal determinants for the risk of asthma.
authors.affiliation: Department of Respiratory Medicine, Bispebjerg Hospital, DK-2400 Copenhagen NV, Denmark.
authors.forename: Karin
authors.initials: K
authors.lastname: Kindlund
authors.forename: Simon Francis
authors.initials: SF
authors.lastname: Thomsen
authors.forename: Lone Graff
authors.initials: LG
authors.lastname: Stensballe
authors.forename: Axel
authors.initials: A
authors.lastname: Skytthe
authors.forename: Kirsten Ohm
authors.initials: KO
authors.lastname: Kyvik
authors.forename: Vibeke
authors.initials: V
authors.lastname: Backer
authors.forename: Hans
authors.initials: H
authors.lastname: Bisgaard
mesh_terms: D000886:Anthropometry / Q000379:methods; D001034:Apgar Score; D001249:Asthma / Q000196:embryology* / Q000453:epidemiology; D001724:Birth Weight; D002648:Child; D002675:Child, Preschool; D003718:Denmark / Q000453:epidemiology; D004200:Diseases in Twins / Q000196:embryology* / Q000453:epidemiology; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic