Presenting with common mental health difficulties, particularly depression and anxiety, there is also preliminary evidence that mindfulness-based interventions (MBIs) including mindfulness-based cognitive therapy (MBCT), mindfulness-based stress reduction (MBSR) and integrated mindfulness yoga practices may also be effective in reducing common mental health difficulties during pregnancy. We systematically reviewed and synthesized the current literature on the effectiveness of MBIs in reducing severity of perinatal anxiety and depression. Databases including PubMed, Cochrane Library, IndMED and PsychoInfo were searched for relevant studies. Manual searches were conducted in relevant articles and Google Scholar. Seventeen cohorts representing 18 studies were included. Pre-post effect sizes were reported for both treatment and control groups. Seven randomized controlled trials (RCTs), two non-randomized controlled trials and nine treatment evaluations were included. Maternal participation in an MBI was associated with reductions in perinatal anxiety of moderate to large magnitude. Results for the effect of MBIs on depression were less consistent, with pre-post treatment reductions of moderate magnitude, but no significant differences in depression scores when MBI was compared with a control group. There was some evidence that MBIs were associated with increased mindfulness. Risk of bias in studies was variable. Our review offers preliminary evidence for the effectiveness of MBIs in reducing perinatal anxiety, with more equivocal findings with regard to perinatal depressive symptoms. Further methodologically rigorous evaluation using RCTs and longer follow-up periods are recommended.