Inborn/outborn status and neonatal survival: a meta-analysis of non-randomized studies

We have applied meta-analysis to investigate the relationship between birth place and the likelihood of neonatal survival, for infants of low birth weight (less than 2501 grams) in a series of 19 non-randomized studies. This paper illustrates the utility meta-analysis in evaluating medical technologies described in non-randomized studies, if proper attention is given to biases in those studies. The results of this meta-analysis show strong preferences for inborn status, especially for infants who weigh 1001-2000 grams. For infants of lower or higher birth weight (that is, less than 1001 or greater than 2000 grams), the studies are inconsistent: some favour inborn status while others favour outborn status. This heterogeneity is not surprising, because selection bias is more problematic in studies of infants at these birth weights. We discuss potential causes of and solutions to selection bias and illustrate its potential magnitude by introducing the bias factor, which should be considered in the design of future studies. When selection bias cannot be ruled out, the results shown for those who weigh 1001-2000 grams are more appropriate for generating valid conclusions and subsequent policies regarding birth place preference for low birth weight infants.