Time seems to be a factor as the risk of failure increases after 1 or more years post-surgery. The risk of ectopic pregnancy is 12.5% for women who have had tubal ligation, which is a greater risk than for those who have not had the surgery. Recanalization or formation of tuboperitoneal fistulas occur, the openings of which are large enough for passage of sperm but too small to allow an ovum to push through, resulting in fertilization/implantation in the distal tubal segment.