Fallopian tubes can become blocked by several different mechanisms. Some of the more common reasons include infections (pelvic inflammatory disease, or PID), surgery, and endometriosis.
Blockage of the fallopian tubes can prevent the sperm and egg from uniting and forming a pregnancy. This blockage can also prevent the normal transport of an embryo and increase the likelihood of an ectopic pregnancy (pregnancy inside the tube). While surgery is still a useful tool to repair damaged tubes, IVF offers a way to bypass the fallopian tubes and increase the probability of a pregnancy with out a more invasive surgery.