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A pregnancy in which the fertilized egg implants in tissue outside of the uterus and the placenta and fetus begin to develop there.

The most common site is within a Fallopian tube. However, ectopic pregnancies can rarely occur in the ovary, the abdomen, and in the lower portion of the uterus (the cervix).

Causes and Risks

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 alter the normal ciliary movement of the tubal epithelium. Slow movement of the fertilized egg down the fallopian tube can result in tubal implantation. Women who become pregnant, despite using progesterone-only oral contraceptives, have a 5-fold increase in the ectopic rate. Progesterone-bearing IUDs increase the risk of ectopic pregnancy from 5% (in non-medicated IUDs) to 15%, and the "morning after pill" is associated with a 10-fold increase in risk (when its use fails to prevent a pregnancy).

The incidence data for ectopic pregnancies ranges from 1 in every 40 to 100 pregnancies. In any case, the incidence of ectopic pregnancy is on the rise (the 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. The incidence in the U.S. is higher in black women than in Caucasian women.