Brazilian women rely on sterilization as the main source of birth control. Sterilization has been one of the causes of the steep decline in fertility in Brazil, at least since the second half of 1970. It is hypothesized that understanding couples’ relationships might be key to explaining this high rate of female sterilizations. Possible reasons for the higher level of fertility among women in unstable unions than among women in stable ones could be the less effective use of contraceptive methods, or that women in unstable unions tend to use less effective or reversible contraceptive methods. In this paper discrete time modelling of the timing of sterilization according to union histories is presented. The analysis uses the calendar data of the 1996 Brazilian DHS. It is shown that women in second or higher order unions have a lower risk of sterilization. This result should be taken into account in the analysis of the determinants of female sterilization in Brazil.