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Evidence of selection bias in preterm survival studies: a systematic review
  1. D J Evans,
  2. M I Levene
  1. Centre for Reproduction, Growth and Development, University of Leeds, D Floor Clarendon Wing, The General Infirmary at Leeds, Leeds LS2 9NS, UK
  1. Dr Evans, Neonatal Intensive Care Unit, Southmead Hospital, Bristol BS10 5NB, UKevans_d{at}


OBJECTIVE To determine by how much selection bias in preterm infant cohort studies results in an overestimate of survival.

DESIGN 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.

CONCLUSION 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).

  • preterm
  • survival
  • selection bias

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