Seventeen trials were included in the review and 15 of these contributed data to the quantitative synthesis. Several trials did not give enough information to assess risk of bias fully. Last study follow-up was at three to six months for all except two trials. Losses to follow-up ranged from 1% to 41% but were generally balanced between groups. Only five trials reported both the number of contacts received and the proportion of women in the intervention group who received some support.
Overall, peer support reduced the risk of not breastfeeding by 15% (RR 0.85, 95% CI 0.77 to 0.94) and of not exclusively breastfeeding by 18% (RR 0.82, 95% CI 0.76 to 0.88). Both effects were statistically significant. Significant heterogeneity was present for both outcomes. In UK RCTs, peer support did not significantly reduce the risk of not breastfeeding (RR 0.96, 95% CI 0.89 to 1.04; three RCTs) and not exclusively breastfeeding (RR 0.98, 95% CI 0.96 to 1.01; two RCTs).
Subgroup and meta-regression analyses showed that peer support had a significantly greater effect in low- or middle-income countries than in high-income countries for both outcomes. Giving support at higher intensity and in the postnatal period had a statistically significant effect for any breastfeeding and not for exclusive breastfeeding.