title: Evidence of selection bias in preterm survival studies: a systematic review.
abstract: OBJECTIVE
  To determine by how much selection bias in preterm infant cohort studies results in an overestimate of survival.
  
  
  METHODS
  Systematic review of studies reporting survival in infants less than 28 weeks of gestation published 1978-1998. Studies were graded according to cohort definition: A, stillbirths and live births; B, live births; C, neonatal unit admissions. Proportions of infants surviving to discharge were calculated for each week of gestation.
  
  
  RESULTS
  Sixty seven studies report data on 55 cohorts (16 grade A, 23 grade B, 16 grade C). Studies that are more selective report significantly higher survival between 23 and 26 weeks of gestation (grade C > grade B > grade A, p < 0.01), exaggerating survival by 100% and 56% at 23 and 24 weeks respectively.
  
  
  CONCLUSIONS
  To minimise the potential for overestimating survival around the limits of viability, future studies should endeavour to report the outcome of all pregnancies for each week of gestation (terminations, miscarriages, stillbirths, and all live births).
authors.affiliation: Centre for Reproduction, Growth and Development, University of Leeds, D Floor Clarendon Wing, The General Infirmary at Leeds, Leeds LS2 9NS, UK. evans_d@southmead.swest.nhs.uk
authors.forename: D J
authors.initials: DJ
authors.lastname: Evans
authors.forename: M I
authors.initials: MI
authors.lastname: Levene
mesh_terms: D015331:Cohort Studies; D005865:Gestational Age; D006801:Humans; D007226:Infant Mortality; D007231:Infant, Newborn; D007234:Infant, Premature; D012107:Research Design; D015983:Selection Bias