this study addressed the process of implementing the first casa de parto birth center in the unified health system in the city of rio de janeiro. the purpose of this qualitative study was to identify the determinants of the process of implementing the birth center and analyze the influence that hegemonic and counter-hegemonic groups have on that process. the theoretical framework used was the concept of hegemony. data analysis was guided by the dialectic method of contradiction, totality and historicity. semi-structured interviews were performed, from january to july 2007, with four municipal health administrators and 11 technical-administrative professionals assigned to implement the birth center. this study showed that the implementation of the birth center was determined by the counter-hegemony established in providing care during pregnancy and physiological deliveries.