Although tubal ligation is considered a permanent form of birth control, the surgical procedure is not completely effective at preventing pregnancy. According to WebMD, within a year after surgery five in 1,000 women are still able to conceive. The rate increases to 13 in 1,000 after five years.
There are several reasons why a tubal ligation sometimes fails to prevent a pregnancy from occurring. In some cases, pregnancy occurs because the surgery itself was not conducted properly. Pregnancy also may occur if the fallopian tubes grow back or a new passage forms that allows an egg to be fertilized by sperm, according to WebMD.