What Causes Ectopic Pregnancies and What Are the Risk Factors?

Ectopic Pregnancy
An ectopic pregnancy can happen to anyone, but sometimes risk factors can make a tubal implantation more likely. Image © A.D.A.M.

Question: What Causes Ectopic Pregnancies and What Are the Risk Factors?

Answer:

Strictly speaking, the cause of an ectopic pregnancy is a fertilized egg implanting somewhere outside the uterus.

In these pregnancies, the implanted zygote/embryo cannot result in a baby being born because implantation in the fallopian tubes - the usual location of an ectopic pregnancy -- would cause the tubes to rupture before the end of the first trimester.

An untreated tubal rupture in an ectopic pregnancy can be fatal.

Numerous risk factors can make an ectopic pregnancy more likely to occur, however, although as with all pregnancy losses, an ectopic pregnancy can occur even in the absence of obvious risk factors.

Most reviews of ectopic pregnancy risk factors divide the list into "high," "moderate," and "low" risk factors based on the strength of the association (i.e., a high risk factor increases the risk of ectopic pregnancy more than a low risk factor).

High Risk Factors

Previous ectopic pregnancy
Women who have had one ectopic pregnancy face a 10% chance of having another.

Abnormal fallopian tubes
Anatomical abnormalities of the fallopian tubes can make implantation in the tubes much more likely than in women without tubal abnormalities.

Maternal DES use
The drug DES (or diethylstilbestrol) has been shown to cause congenital abnormalities of the uterus in girls born to mothers who took the drug during pregnancy.

The fallopian tubes in these girls can also be formed in a way that makes ectopic pregnancy more likely. Doctors stopped prescribing DES to pregnant women in the early 1970s.

Endometriosis
Women who have endometriosis have an increased risk of ectopic pregnancy because the disorder increases the chance of scar tissue and adhesions that interfere with the ability of the fertilized egg to reach the uterus.

History of Tubal Surgery
Having had surgical procedures involving the fallopian tubes, such as tubal ligation, can make an ectopic pregnancy more likely. Having a previous tubal ligation means that a conception has a 1 in 3 chance of resulting in an ectopic pregnancy.

Use of IUD device
Contraceptive intrauterine devices have long been considered a risk factor for ectopic pregnancy, but researchers believe that IUDs do not technically increase the risk of ectopic pregnancy -- but if a conception occurs, the pregnancy has an increased risk of being ectopic. Women who use IUDs have a lower risk of ectopic pregnancy than women who use no birth control at all. If a conception occurs with a Mirena in place, there is a 1 in 2 chance that it will be an ectopic pregnancy and with a Paragard, it has a 1 in 16 chance of being an ectopic pregnancy.

Moderate Risk Factors

History of sexually transmitted infections or pelvic inflammatory disease
Sexually transmitted diseases can lead to pelvic inflammatory disease.

History of infertility
Some medical factors that cause infertility might also make ectopic pregnancy more likely, and researchers also believe that the drugs commonly used to treat infertility might also increase ectopic pregnancy risk.

Multiple sexual partners
The reason why having multiple sexual partners increases risk is most likely due to an increased chance of acquiring a sexually transmitted infection.

Exposure to cigarette smoke
The greater the exposure to smoke, the higher the risk for ectopic pregnancy. One study found that women who smoked 20 or more cigarettes per day were almost four times more likely to have an ectopic pregnancy than women who never smoked.

Low Risk Factors

Douching
Some doctors think that douching could potentially cause abnormal bacteria present in the vagina to ascend higher in the reproductive tract and lead to inflammation of the tubes.

Past abdominal surgery
In one study, women who had an appendectomy for a ruptured appendix had an increased risk of miscarriage.

Age
Risk of an ectopic pregnancy appears to increase for older moms, with moms over 40 having the highest risk.

Sources:

Bouyer, Jean, Joel Coste, Taraneh Shojaei, Jean-Luc Pouly, Herve Fernandez, Laurent, Gerbaud, and Nadine Job-Spira, "Risk Factors for Ectopic Pregnancy: A Comprehensive Analysis Based on a Large Case-Control, Population-based Study in France." American Journal of Epidemiology 2003. Accessed 21 Apr 2008.

Tenore, Josie L., "Ectopic Pregnancy." American Family Physician 15 Feb 2000. Accessed 21 Apr 2008.

Tulandi, Togas, "Incidence, risk factors, and pathology of ectopic pregnancy." UpToDate for Patients Jan 2008. Accessed 21 Apr 2008.

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