This paper is a consequence of my professional insertion as a social assistant in an emergency room unity of Rio de Janeiro State - Hospital Estadual Carlos Chagas - HECC, where I met several children who did not have a birth certificate. This fact surprised me even more when I realized the situation was significantly recurrent, as well as its reverberations. I noticed that these children were referenced by HECC staff as Child of, plus the name of their mothers. Slowly I understood that the birth certificate absence constitutes a phenomenon officially called birth under-registration, about what there is a few theoric production, as I could verify during the investigation process. This issue is my researchs subject. My goal with this paper was to understand why some children treated in HECC during the period between 1999 and 2007 were not registered. Seeking for an ethnographic approach, my objective was to find out the reasons for that fact. In order to do that, I have investigated the meanings of birth certificates with mothers and fathers, and their implication on interactions with educational institutions. Since some children were not registered because their mothers were not registered neither, I have incorporated the adults with no birth certificate in the investigation process. This issue suggests me an intriguing question for the debate about civil register which involves a social reproduction of society poor sectors. With this paper, I intend to contribute for the understanding of childhood background and Brazilian`s poor families, expecting that this investigation is an alert for the sectors which are responsible for the civil register system in Brazil. In addition, I intend to contribute for the elaboration of educational public policies. As working tools, I used a questionnaire, open interviews, conversations with mothers and fathers, documents and institutional material from HECC. My central dialogue occurred with Roberto DaMatta (1997a; 1997; 2000), Ana Liési Thurler (2004); and Cynthia Sarti (1996;2004) e Cláudia Fonseca (2005; in edition).