Cesarean section rates are very high in Brazil mainly in private hospitals, probably due to socioeconomic and cultural factors. The objective of this study was to describe socioeconomic, demographic, cultural and reproductive characteristics of women in the postpartum period and the factors that had determined their decision for caesarean section in two units of the supplementary health care system of the State of Rio de Janeiro. The study population was composed of 437 women that had vaginal or caesarean childbirths in the two selected units. Data were collected by means of interviews with mothers and consultation of hospital records. The factors associated with the decision for cesarean section as mode of delivery were evaluated using non-conditional logistic regression analysis and following the hierarchic models established at three definite moments. Although 70% of the women had no initial preference for cesarean section, 90% of them had this mode of birth. It was verified that, despite their initial desire, the interaction with the health services resulted in cesarean section as mode of birth. Educative actions directed to pregnant women and to the public at large as well as changes in the childbirth care model can be promising strategies for reverting this picture.