The exact mechanism responsible for an ectopic pregnancy following an IVF procedure is unknown.
Some believe that embryo migration up into the fallopian tubes occurs because of local cellular activity or fluid mechanics present inside the uterus. Sometimes the opening of the fallopian tube in the uterus is dilated because of disease, making it easier for the embryos to enter the tubes.
As described in Part 3, an ectopic pregnancy can occur within the section of the fallopian tube that passes through the muscle of the uterus or within the short segment of fallopian tube that remains after surgical removal of the tube. The incidence of ectopic pregnancy following IVF ranges from 0.5 % to 3%, but this figure may be decreasing. For the past several years, embryo transfer has been routinely performed using ultrasound to properly guide the embryo catheter to the optimal uterine location, which may help to reduce the risk of an ectopic pregnancy. However, even ultrasound guided embryo transfer cannot eliminate the possibility of an ectopic pregnancy after IVF.
Here are more resources for frozen embryo transfer procedures.