Non-surgical tubal occlusion

Out of 151 women provided with chemical female sterilization with 3 insertions of 250 mg of quinacrine spaced 1 month apart, there were 7 pregnancies in 3 years. If these pregnancies were a result of failure to occlude the entrance to 1 or both fallopian tubes, birth events should decrease exponentially with time. Unexpectedly, the low failure rate has not decreased significantly over the 3-year observation period.