Background: Mental health problems are frequent in the postpartum period, but accessible treatment options are lacking. The MPOWER study investigated whether the use of trained lay coaches could increase the uptake and effectiveness of a web-based intervention (WBI) for women with postpartum depression and/or anxiety., Objectives: First, to compare the feasibility and acceptability of a WBI for women with postpartum depression and anxiety, with and without the addition of telephone coaching calls. Second, to estimate the effectiveness of the WBI at decreasing symptoms of depression and anxiety at 6 months, with and without coaching calls., Methods: We conducted a pilot randomized controlled trial (RCT) that enrolled women who had recently given birth and had mild to moderate postpartum depression and/or anxiety. Study participants were provided access to the WBI. Women randomized to the intervention group also received up to 7 telephone coaching calls during the 6 months of follow up. We evaluated the feasibility of the intervention through participants' usage of the WBI, as well as the completion and fidelity of planned coach calls. We measured acceptability via two questionnaires on the usability of the WBI and participant satisfaction with the intervention. To determine the potential effectiveness of the intervention, the primary outcomes were defined as symptoms of depression and anxiety at 6 months and adjusted mean differences between groups for these outcomes were estimated using linear regression models., Results: We recruited 52 participants (25 intervention; 27 control). At 6 months, 88 % (22/25) of participants randomized to the intervention arm and 59 % (16/27) of participants randomized to the control arm remained in the study. The intervention group had an average of 11 (95 % CI: [5, 18]) more website logins than the control group. Intervention group participants completed a mean of 6.2 coaching calls with high fidelity. The estimates of the effect of the intervention on mental health outcomes at 6-months were imprecise but point estimates and confidence intervals were consistent with a moderate beneficial effect of the intervention on both symptoms of depression and anxiety (fully adjusted effects sizes: 0.51 (95 % CI: [-0.14, 1.17]) and 0.56 (95 % CI: [-0.09, 1.22]), respectively)., Conclusions: WBIs with coaching are feasible, acceptable, and potentially effective treatment options for women with mild to moderate postpartum depression and/or anxiety. The addition of coaching calls markedly increased engagement with the WBI, but a larger RCT is needed to determine the effectiveness of such an intervention., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 The Authors.)