Ectopic pregnancies are usually caused by conditions that obstruct or slow the passage of a fertilized ovum (egg) through the Fallopian tube to the uterus. This may be caused by a physical blockage in the tube, or by failure of the tubal epithelium to move the zygote (the cell formed after the egg is fertilized) down the tube and into the uterus.

Most cases are a result of scarring caused by previous tubal infection or tubal surgery. Up to 50% of women with ectopic pregnancies have a medical history of salpingitis or PID (pelvic inflammatory disease). Some ectopic pregnancies can be traced to congenital tubal abnormalities, endometriosis, tubal scarring and kinking caused by a ruptured appendix, or scarring caused by previous pelvic surgery and prior ectopic pregnancies. In a few cases, the cause is unknown.

On occasion, a woman will conceive after elective tubal sterilization. The risk of an ectopic pregnancy occurring in this situation may reach 60%. Women who have had surgery to reverse previous tubal sterilization in order to become pregnant also have an increased risk of ectopic pregnancy (when reversal is successful).

The administration of hormones (specifically estrogen and progesterone) can slow the normal movement of the fertilized egg through the tubal epithelium and result in implantation in the tube. Women who become pregnant despite using progesterone-only oral contraceptives have a 5-fold increase in the ectopic pregnancy rate.

Women who become pregnant despite using progesterone-bearing IUDs also have an increased risk of ectopic pregnancy. Ectopic pregnancy rates for those who become pregnant despite non-medicated IUD are 5%, while the rate for medicated IUD users who become pregnant despite the device is 15%. Note that these rates only refer to percents of the tiny proportion of women who become pregnant while using these methods -- they do not refer to women who have once used these methods and later become pregnant, or to the percent of women who become pregnant while using these methods.

The "morning after pill" is associated with a 10-fold increase in risk of this condition when its use fails to prevent pregnancy.

Ectopic pregnancies occur from 1 in every 40 to 1 in every 100 pregnancies. This rate increased four-fold between 1970 and 1992.

Increased risk is associated with women who have a history of salpingitis or PID, tubal surgery of any type (including tubal ligation and reversal of), or prior ectopic pregnancy.