Sixteen studies were included in the review (n=982,579, range 206 to 60,1883). There were 12 analyses of induced abortion (eight cohort, n=313,313, and four case-control, n=11,498) and nine of spontaneous abortion (six cohort, n=856,014 and three case-control, n=1,626).
Induced abortion: The risk of preterm birth increased modestly following one or more induced abortions (adjusted OR 1.32, 95% CI 1.11 to 1.53; 10 studies). The OR increased with multiple induced abortions and ranged from 1.25 (95% CI 1.03 to 1.48) following one abortion to 1.51 (95% CI 1.21 to 1.75) following two or more abortions. The risk of more marked preterm birth associated with induced abortion was 1.64 (95% CI 1.38 to 1.91; four studies). Results from unadjusted data were similar. Statistical testing showed heterogeneity among the studies.
Spontaneous abortion: The risk of preterm birth increased following one spontaneous abortion (adjusted OR 1.43, 95% CI 1.05 to 1.66; six studies) and following more than one spontaneous abortion (adjusted OR 2.27, 95% CI 1.98 to 2.81; seven studies).
Meta-regression showed a statistically significant inverse relationship between the baseline preterm birth rate and the risk associated with either type of abortion. ORs were higher in populations with lower baseline preterm birth rates. This explained some of the heterogeneity between the studies.
The funnel plot did not show evidence of significant publication bias.