Abortion and the risk of subsequent preterm birth: a systematic review with meta-analyses.

OBJECTIVE: To conduct a systematic review and meta-analyses of studies that test the association between induced or spontaneous abortion and subsequent preterm birth.

STUDY DESIGN: International databases were reviewed (1995-2007) using the terms preterm, premature, birth, labor, delivery, abortion, induced abortion, miscarriage and spontaneous abortion. Only studies that met prespecified objective criteria for methodologic design and reporting were included in the meta-analyses.

RESULTS: Twelve induced and 9 spontaneous abortion studies met inclusion criteria. Common adjusted odds ratios (ORs) for preterm birth following 1 and > or = 2 induced abortions were 1.25 (95% confidence interval [95% CI] 1.03-1.48) and 1.51 (95% CI 1.21-1.75), respectively. Four studies provided a common adjusted OR for < or = 32 weeks' births of 1.64 (95% CI 1.38-1.91). Meta-regression analysis revealed a previously unrecognized inverse relationship between the In OR and the control population preterm birth rate, explaining in part the observed heterogeneity among studies. Analysis of spontaneous abortion and subsequent preterm birth revealed a similar common adjusted OR and inverse meta-regression on the control preterm birth rates.

CONCLUSION: Induced and spontaneous abortion are associated with similarly increased ORs for preterm birth in subsequent pregnancies, and they vary inversely with the baseline preterm birth rate, explaining some of the variability among studies.