Given the methodological limitations of recently published qualitative reviews of abortion and mental health, a quantitative synthesis was deemed necessary to represent more accurately the published literature and to provide clarity to clinicians.

To measure the association between abortion and indicators of adverse mental health, with subgroup effects calculated based on comparison groups (no abortion, unintended pregnancy delivered, pregnancy delivered) and particular outcomes. A secondary objective was to calculate population-attributable risk (PAR) statistics for each outcome.

After the application of methodologically based selection criteria and extraction rules to minimise bias, the sample comprised 22 studies, 36 measures of effect and 877 181 participants (163 831 experienced an abortion). Random effects pooled odds ratios were computed using adjusted odds ratios from the original studies and PAR statistics were derived from the pooled odds ratios.

Women who had undergone an abortion experienced an 81% increased risk of mental health problems, and nearly 10% of the incidence of mental health problems was shown to be attributable to abortion. The strongest subgroup estimates of increased risk occurred when abortion was compared with term pregnancy and when the outcomes pertained to substance use and suicidal behaviour.

This review offers the largest quantitative estimate of mental health risks associated with abortion available in the world literature. Calling into question the conclusions from traditional reviews, the results revealed a moderate to highly increased risk of mental health problems after abortion. Consistent with the tenets of evidence-based medicine, this information should inform the delivery of abortion services.