The established efficacy in postpartum lifestyle interventions has not been translated into better outcomes. This systematic review and meta-analysis assess the penetration (the proportion of women invited within the target population), implementation (fidelity), participation (the proportion of those invited who enrolled), and effect (weight loss compared to controls) (PIPE) of randomized controlled trials of lifestyle interventions in postpartum women (within two years after birth). MEDLINE, EMBASE, Pubmed, and other databases and clinical trial registries were searched up to the 3rd of May 2019. Data was extracted from published reports and missing data was obtained from study authors. The quality of the studies was appraised using the Cochrane Risk of Bias tool (2 center dot 0). Main outcomes were the PIPE impact metrics and changes in body weight. Thirty-six trials (49 publications) were included (n=5,315 women). One study provided sufficient information to calculate the population penetration rate (2 center dot 5%). All studies provided implementation (fidelity) information, but over half had low program fidelity. The participation rate was calculated for nine studies (0 center dot 94% to 86%). There was significant change in body weight (mean difference (MD) (95% confidence interval, CI) of -2 center dot 33 (-3 center dot 10 to -1 center dot 56). This highlights the inadequacy of conventional RCTs to inform implementation. Future research should broaden methods to pragmatic trials.