title: Vaginal bleeding in early pregnancy and preterm birth: systemic review and analysis of heterogeneity.
abstract: OBJECTIVE
  To systemically review published studies of vaginal bleeding and the risk of preterm birth (PTB) and explore sources of heterogeneity between them.
  
  
  METHODS
  The literature was searched for peer-reviewed articles from 1980 to 2009 in which the primary analysis was the risk of PTB among low-risk subjects with and without bleeding. Heterogeneity was assessed through I(2) statistics, and sources of heterogeneity were explored through subgroup analyses and meta-regression.
  
  
  RESULTS
  218 studies were initially identified, 64 reviewed and 23 included. The pooled Odds Ratio for PTB was 1.74, though significant heterogeneity was present (I(2)  =  49.7%). Meta-regression demonstrated a significant association between a study's incidence of bleeding and quality assessment and subsequent odds ratio, such that studies with a lower quality assessment or lower incidence of bleeding demonstrated an increased odds of PTB.
  
  
  CONCLUSIONS
  Bleeding in early pregnancy is associated with an increased risk of PTB; however, excessive heterogeneity exists among published studies. The heterogeneity arises in part from differences in the reported incidence of bleeding within study populations. Presumably studies that identify bleeding in a larger percentage of subjects consequently dilute the magnitude of the risk.
authors.affiliation: Department of Obstetrics and Gynecology, University of Rochester School of Medicine, Rochester, New York, USA. david_hackney@URMC.rochester.edu
authors.forename: David N
authors.initials: DN
authors.lastname: Hackney
authors.forename: J Christopher
authors.initials: JC
authors.lastname: Glantz
mesh_terms: D005260:Female; D006801:Humans; D007231:Infant, Newborn; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology; D047928:Premature Birth / Q000453:epidemiology / Q000209:etiology; D012307:Risk Factors; D014592:Uterine Hemorrhage / Q000150:complications / Q000453:epidemiology; D014623:Vaginal Diseases / Q000150:complications / Q000453:epidemiology