Introduction: Inadequate birth weight is an important indicator of infant morbidity and mortality, and its assessment is essential to develop strategies aimed at improving maternal and child health especially among vulnerable populations. It is not clear the effect of strategies already used, such as the Bolsa Família Program, in reducing the inadequate birth weight in Brazil. Objectives: The present study has three objectives: 1. To investigate the  factors associated with TLBW in the Brazilian population living in poverty and extreme poverty; 2.  To estimate the occurrence of and sociodemographic factors associated with Small (SGA) and Large-for-gestational-age (LGA) births in the poor and extremely poor population of Brazil; and 3. To demonstrate an analysis protocol to assess the impact of receiving Bolsa Família on birth weight. Methods: These are studies with a retrospective and dynamic cohort. The cohort database contains records of individuals eligible for social assistance programs via the Unified Registry for Social Programs (CadÚnico), who were registered between 2001 and 2015. Socioeconomic data from the Cohort 100 Million Brazilians were linked to data contained in the National System Information on Live Births (SINASC), considering the period from Jan 1, 2001 to Dec 31, 2015. The study population of the first two articles included most recent live births born at term (37-41 gestational weeks) from women aged 14-49 years who entered in the 100 Million Brazilian Cohort between 2001-2015. For the second article, only births from 2012 to 2015 were considered. The population for impact analysis, present in research protocol, will include of the first live birth from women aged 10 to 49 years, who entered in the 100 Million Brazilian Cohort between 2004-2015. Multivariate logistic regression was performed in articles 1 and 2. A hierarchical conceptual approach was employed. In the impact assessment protocol, we propose methods of analysis based on the propensity score. The average treatment effect on the treaties (Average Treatment effect for the Treated - ATT) will be calculated considering the different metrics based on the linear (birth weight) and non-linear outcome scale (low birth weight, small and large for gestational age). Results: The first article indicated that the highest odds of low weight at term were associated with female newborns (OR: 1.49; 95% CI: 1.47-1.50), whose mothers were black (OR: 1.20; 95% CI: 1.18-1.22), had a low educational level (OR: 1.57; 95% CI: 1.53-1.62), were aged ≥35 years (OR: 1.44; 95% CI: 1.43-1.46), had a low number of prenatal care visits (OR: 2.48; 95% CI: 2.42- 2.54) and were primiparous (OR: 1.62; 95% CI: 1.60-1.64). Among the results of the second article, highest odds of SGA were associated among those children born to women who self-reported as black (OR: 1.21; 95% CI: 1.20-1.23), mixed/parda (OR: 1.08; 95% CI: 1.07; 1.09) or indigenous (OR: 1.10; 95% CI: 1.05-1.14), were unmarried  (OR: 1.09; 95% CI: 1.08-1.10), illiterate (OR: 1.49; 95%CI: 1.43-1.55), did not attend prenatal consultations (OR: 1.62; 95%CI: 1.58-1.66) or were aged 14-20 years (OR: 1.26; 95% CI: 1.24-1.27) or 35-49 years (OR: 1.11; 95% CI: 1.10-1.13). Considering LGA children, higher odds were found among those born to women living in municipalities with low/very low Municipal Human Development Index (OR: 1.23; 95% CI: 1.22-1.24) and among older women (OR: 1.31; 95% CI: 1.30-1.32). However, the odds of LGA decreased for infants born to younger women who attended fewer prenatal visits. The Bolsa Família Program (BFP) impact assessment proposal was detailed in the research protocol. The protocol followed internationally recognized guidelines for conducting and disseminating the results of impact assessment studies, providing transparency in conducting data analysis and greater comparability of results. The availability of a cohort with a large amount of explanatory and confounding variables makes it possible to evaluate the effect of BFP in birth weight, considering the use of methods based on propensity score. Conclusion: Multiple aspects were associated with low birth weight at term, SGA and LGA, highlighting the need to examine comprehensively the mechanisms underlying these factors. The PBF impact evaluation proposal may provide evidence to justify the use of conditional cash transfer programs to reduce disparities related to inadequate birth weight.