We conducted a case-control study, in which the cases represented pregnant women who presented a birth plan and the controls represented pregnant women who did not present a birth plan. Sample size was determined by a power analysis using the Epitable program from EpiInfo and taking into account the following data: the main out-come (type of birth), two to three controls per case, odds ratio value of 3, 20% exposure between controls, 80% power, and a confidence interval of 95%