What causes ectopic pregnancies?

The major cause of ectopic pregnancy is salpingitis, accounting for about half of first time ectopic pregnancies. In most of the remaining first time ectopic pregnancies (accounting for about 40%), the cause is undetermined. One theory is that in some women the fertilized egg travels in the Fallopian tube slower, so that at time of implantation (7 days after fertilization) the embryo is still in the Fallopian tube. Instead of implanting in the uterus as is normal, it implants in the Fallopian tube. A possible cause for this slowing down of travel is a hormonal imbalance. Increased amount of estrogen or progesterone can change the contractions of the tube (contractions are required to help move or propel the embryo along the tube). Increased levels of progesterone can be found with the use of a progesterone-releasing IUD (intrauterine device, for contraception) or the use of progestin-only oral contraceptives (one type of "the pill"). Increased levels of both estrogen and progesterone can occur after hormonal treatment for the purpose of in-vitro fertilization ("test tube baby").

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Another theory is that an abnormality of the embryo, such as an incorrect number of chromosomes (normally every cell has 23 pairs of chromosomes, and 1 X and 1 Y chromosome; these chromosomes carries the person's complete genetic material, which should be the same in each cell), would interfere with its ability for normal transport in the Fallopian tube. Many studies have indicated that cigarette smoking increases the risk of ectopic pregnancy. The greater the number of cigarettes smoked per day, the greater the risk. Smoking 30 or more cigarettes per day increases a woman's risk 4 times. 7% of women with ectopic pregnancy have had an ectopic pregnancy before. After one ectopic pregnancy, about 12% of subsequent pregnancies are ectopic pregnancies.

Summary: The common cause of ectopic pregnancy is salpingitis (causes 50% of first time ectopic pregnancies). 40% have no known cause. One hypothesis is that there is slowing of transport through the Fallopian tube of the fertilized egg. This transport slowly could possibly be due to hormonal imbalance (such as due to progesterone releasing IUD, progestin-only oral contraceptive or in-vitro fertilization treatment) or possibly an abnormality (e.g. chromosomal) of the embryo. Cigarette smoking significantly increases a woman's risk. Previous ectopic pregnancy increases the risk for another.

How does salpingitis cause ectopic pregnancy?

Salpingitis can fuse together the folds in the Fallopian tube. These folds are naturally found lining the inside of the tube. This narrows the inside of the tube such that sperms can travel normally through it, but the embryo cannot. Secondly, the embryo can be trapped in blind pockets formed by adhesions inside the tube (adhesions are abnormal joining between organ parts, which usually form after damage to organs).

Summary: Salpingitis can prevent normal transport of the embryo by (1) narrowing the Fallopian tube by fusing together folds inside the tube, and/or (2) trapping the embryo in blind pockets formed by adhesions inside the tube.

How about other abnormalities to the tubes?

Besides salpingitis from infection, previous surgery is another cause of adhesions. About one quarter of women with ectopic pregnancy had previously had surgery in the abdomen. Another abnormality that can be found is diverticulum of the Fallopian tube, which is small out-pouchings of the tube.

Summary: About 25% of women with an ectopic pregnancy had previous surgery in the abdomen.

Can “tying the tubes” cause ectopic pregnancies?

It can occur, but rarely. The chance of unexpectedly getting pregnant after tubal sterilization (failure of sterilization) is about 2%. Of these pregnancies, 40% are ectopic. In sterilization done by electrocoagulation ("melting and solidification" of tissue with electricity) a passage can occasionally be created for sperms to exit the uterus into the pelvic cavity through the site of coagulation. The sperm can then travel to the fimbrial end of the Fallopian tube to fertilize an ovulated egg (see diagram below) and cause an ectopic pregnancy. Women sterilized with metal clips or silicone rings are also at risk. Click on image for diagram in new window. If a woman who has undergone tubal sterilization misses an expected menses (period), a pregnancy test should be done immediately. If pregnant, it is necessary to determine if it is ectopic.

Summary: Failure of tubal sterilization resulting in an ectopic pregnancy rarely occurs (less than 1%). This can be due to exit of the sperm into the pelvic cavity and subsequent entry of the Fallopian tube through the fimbrial end to fertilize an egg. If a woman who has undergone tubal sterilization misses an expected menses, a pregnancy test should be done immediately.