title: Breast-feeding and health consequences in early childhood: is there an impact of time-dependent confounding?
abstract: BACKGROUND
  Estimated effects of breast-feeding on childhood health vary between studies, possibly due to confounding by baseline maternal and child characteristics. Possible time-dependent confounding has received little consideration. Our aim was to evaluate the impact of such confounding.
  
  
  METHODS
  We estimated the relationship between cumulative exclusive breast-feeding up to 6 months and wheezing, rash and body mass index (BMI) at 12 months [in the Whistler cohort (n = 494) and PROBIT (n = 11,463)], and wheezing, rash, asthma, hay fever, eczema, allergy and BMI at age 6.5 years (PROBIT). We adjusted for time-dependent confounding by weight, length, rash, respiratory illness and day care attendance using marginal structural models (MSMs).
  
  
  RESULTS
  Weight and day care attendance appeared potential time-dependent confounders, since these predicted breast-feeding status and were influenced by previous breast-feeding. However, adjustment for time-dependent confounders did not markedly change the estimated associations. For example, in PROBIT the adjusted increase in BMI at 12 months per 1-month increase in exclusive breast-feeding was 0.04 (95% CI -0.09 to 0.01) using logistic regression and -0.06 (95% CI -0.11 to -0.01) using MSM. In Whistler, these estimates were each -0.05 (95% CI -0.10 to 0.00).
  
  
  CONCLUSIONS
  In two cohort studies, there was little evidence of time-dependent confounding by weight, length, rash, respiratory illness or day care attendance of the effects of breast-feeding on early childhood health.
authors.affiliation: Julius Center for Health Sciences and Primary Care, Wilhelmina Children's Hospital, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: R H H
authors.initials: RH
authors.lastname: Groenwold
authors.forename: K
authors.initials: K
authors.lastname: Tilling
authors.forename: K G M
authors.initials: KG
authors.lastname: Moons
authors.forename: A W
authors.initials: AW
authors.lastname: Hoes
authors.forename: C K
authors.initials: CK
authors.lastname: van der Ent
authors.forename: M S
authors.initials: MS
authors.lastname: Kramer
authors.forename: R M
authors.initials: RM
authors.lastname: Martin
authors.forename: J A C
authors.initials: JA
authors.lastname: Sterne
mesh_terms: D001249:Asthma / Q000453:epidemiology; D015992:Body Mass Index; D001835:Body Weight; D001942:Breast Feeding; D002648:Child; D002675:Child, Preschool; D016000:Cluster Analysis; D005076:Exanthema / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D006967:Hypersensitivity / Q000453:epidemiology; D007223:Infant; D016015:Logistic Models; D008297:Male; D012135:Respiratory Sounds; D006255:Rhinitis, Allergic, Seasonal / Q000453:epidemiology; D012307:Risk Factors