Introduction: While a number of randomised controlled trials (RCTs) have investigated interventions to reduce postpartum weight retention, little is known about the impact of these interventions. This is a systematic review to assess the penetration, implementation, participation and effectiveness (PIPE) of lifestyle interventions in postpartum women. Methods: We searched 5 databases to identify lifestyle intervention RCTs in postpartum women (within 2 years post-delivery) published up to January 2018. Program impact was evaluated according to the Penetration, Implementation, Participation and Effectiveness (PIPE) impact metric. Results: 33 trials (42 publications) were included (n = 4960 women, age 18-34 years). Only one study provided sufficient information for population penetration rate calculation (2.5%). Participation rate was calculated for five studies (0.94% to 86%). All studies provided implementation information but over half had low program fidelity due to no reported intervention manual or structured curriculum, checklist or other measures of quality-of-assurance. Effect was reported in twenty-three studies with a pooled mean difference (MD) (95% confidence interval, CI) of -2.21 (-2.98 to -1.43) kg change in body weight (23 RCTs, 1781 participants). Lifestyle intervention also resulted in significant improvements in BMI (MD -0.96 kg/m2, 95% CI -1.42 to -0.51, 15 studies, 748 participants, I2 = 63%), energy intake (MD -697.20 [95% CI -1220.29, -174.10, 11 RCTs, 846 participants]) and physical activity (SMD 0.62 [95% CI 0.32, 0.91, 19 RCTs, 1773 participants]). Conclusion: The lack of reporting, and possibly collection of, population penetration and participation data in RCTs is concerning and the limited evidence suggests very low population impact. Despite large numbers of efficacy RCTs being conducted in postpartum women, no useful information exists to inform the implementation of weight management in this population. This work highlights the inadequacy of RCTs in the development of complex interventions suitable for implementation. Future research should consider additional methods to deliver real-world, implementable solutions to postpartum weight management.