Over the 3-year study period , 0.33 pregnancies per 100 women-years ( 95 % confidence interval [ CI ] 0.16-0 .60 ) were observed with the 13.5 mg intrauterine contraceptive system compared with 0.31 per 100 women-years ( 95 % CI 0.15-0 .57 ) with the 19.5 mg intrauterine contraceptive system .