Fourteen studies were identified (1,396 participants, range 16 to 300): 13 RCTs and one quasi-experimental study (118 participants). The RCTs all allocated interventions to particular health institutions; subjects were not subsequently randomised to different interventions. Mean study quality was 10.6 points (range 8 to 13). Eight studies reported data at follow-up (range one to 24 months, most commonly six or 12 months).
The overall pooled effect size was significant (g 0.31, 95% CI 0.17 to 0.45, Ι²=32.9%; 14 studies), estimated as small to moderate. Six times the number of studies would be needed to reduce the finding to a null effect (failsafe N). The effect size was still significant after removal of one outlier from the analysis (g 0.27, 95% CI 0.15 to 0.38).
Subgroup and meta-regression: Mean age of participants (14 studies), marital status (13 studies) and prenatal versus other timing were not significantly related to effect size. The effect was significant for prenatal (seven studies) and postnatal studies (seven studies).
Intervention characteristics: Psycho-education (seven studies) and social support interventions (three studies) both had significant effects but interpersonal therapy was not significant (four studies). Both individual (five studies) and group (nine studies) delivery and both community centred (nine studies) and home-based (five studies) interventions had a significant effect. There was no relation between effect size and intervention duration (14 studies). Studies with no booster sessions (10 studies) had a significant effect and those with boosters (four studies) did not. Effects were significantly greater with delivery by nurse (three studies) and trained non-professionals (three studies) than for mental health professionals (eight studies) but the difference was no longer significant when an outlier study was removed from the analysis.
Study characteristics: There was no significant relationship between effect size and study quality (14 studies) and no difference for studies that did or did not exclude participants with depression at baseline or where control groups were care as usual (nine studies), information only (four studies) and alternative interventions (one study).
Effects over time: There were no significant differences in effect size post intervention or at follow-up. Effects were not significant at six-month (five studies) and 12-month (four studies) follow-up. Overall effect size at follow-up showed a trend that declined over time (eight studies).
There was no evidence for publication bias.