title: Re-evaluation of link between interpregnancy interval and adverse birth outcomes: retrospective cohort study matching two intervals per mother.
abstract: OBJECTIVE
  To re-evaluate the causal effect of interpregnancy interval on adverse birth outcomes, on the basis that previous studies relying on between mother comparisons may have inadequately adjusted for confounding by maternal risk factors.
  
  
  METHODS
  Retrospective cohort study using conditional logistic regression (matching two intervals per mother so each mother acts as her own control) to model the incidence of adverse birth outcomes as a function of interpregnancy interval; additional unconditional logistic regression with adjustment for confounders enabled comparison with the unmatched design of previous studies.
  
  
  METHODS
  Perth, Western Australia, 1980-2010.
  
  
  METHODS
  40 441 mothers who each delivered three liveborn singleton neonates.
  
  
  METHODS
  Preterm birth (<37 weeks), small for gestational age birth (<10th centile of birth weight by sex and gestational age), and low birth weight (<2500 g).
  
  
  RESULTS
  Within mother analysis of interpregnancy intervals indicated a much weaker effect of short intervals on the odds of preterm birth and low birth weight compared with estimates generated using a traditional between mother analysis. The traditional unmatched design estimated an adjusted odds ratio for an interpregnancy interval of 0-5 months (relative to the reference category of 18-23 months) of 1.41 (95% confidence interval 1.31 to 1.51) for preterm birth, 1.26 (1.15 to 1.37) for low birth weight, and 0.98 (0.92 to 1.06) for small for gestational age birth. In comparison, the matched design showed a much weaker effect of short interpregnancy interval on preterm birth (odds ratio 1.07, 0.86 to 1.34) and low birth weight (1.03, 0.79 to 1.34), and the effect for small for gestational age birth remained small (1.08, 0.87 to 1.34). Both the unmatched and matched models estimated a high odds of small for gestational age birth and low birth weight for long interpregnancy intervals (longer than 59 months), but the estimated effect of long interpregnancy intervals on the odds of preterm birth was much weaker in the matched model than in the unmatched model.
  
  
  CONCLUSIONS
  This study questions the causal effect of short interpregnancy intervals on adverse birth outcomes and points to the possibility of unmeasured or inadequately specified maternal factors in previous studies.
authors: Telethon Kids Institute, University of Western Australia, PO Box 855, West Perth, WA 6872, Australia stephen.ball@telethonkids.org.au.
authors: Stephen J
authors: SJ
authors: Ball
authors: Telethon Kids Institute, University of Western Australia, PO Box 855, West Perth, WA 6872, Australia Yale Center for Perinatal, Pediatric and Environmental Epidemiology, Yale University, New Haven, CT 06510, USA.
authors: Gavin
authors: G
authors: Pereira
authors: Telethon Kids Institute, University of Western Australia, PO Box 855, West Perth, WA 6872, Australia.
authors: Peter
authors: P
authors: Jacoby
authors: Telethon Kids Institute, University of Western Australia, PO Box 855, West Perth, WA 6872, Australia.
authors: Nicholas
authors: N
authors: de Klerk
authors: Telethon Kids Institute, University of Western Australia, PO Box 855, West Perth, WA 6872, Australia.
authors: Fiona J
authors: FJ
authors: Stanley
mesh_terms: D000293:Adolescent; D000328:Adult; D001315:Australia / Q000453:epidemiology; D001721:Birth Intervals; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D007236:Infant, Small for Gestational Age; D016015:Logistic Models; D008297:Male; D016017:Odds Ratio; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology*; D012189:Retrospective Studies; D055815:Young Adult