Vaginal bleeding in early pregnancy and preterm birth: systemic review and analysis of heterogeneity.

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.