What are the Causes and Risk Factors for Ectopic Pregnancy?

High, Moderate, and Low Risk Factors for a Tubal Pregnancy

Close up of ectopic pregnancy
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What causes an ectopic pregnancy and what are the risk factors? What are the differences between high risk factors, moderate risk factors, and low risk factors for a tubal pregnancy?

What Causes an Ectopic (Tubal) Pregnancy (The Mechanism)

Strictly speaking, the cause of an ectopic pregnancy is a fertilized egg implanting somewhere outside the uterus. Implantation takes place around nine days after ovulation.

In an ectopic or tubal pregnancy, implantation of the zygote/embryo most often takes place in the fallopian tubes. Since the growth of an ectopic pregnancy in the fallopian tubes would rupture the tubes before the end of the first trimester, the pregnancy cannot result in the birth of a baby. In fact, an untreated ectopic pregnancy is a medical emergency, and can be fatal if it ruptures without prompt treatment. Thankfully, an awareness of tubal pregnancies and good medical care has resulted in much better outcomes now than in the past.

Risk Factors for Ectopic Pregnancy

There are several risk factors for an ectopic pregnancy, but as with other forms of pregnancy loss, an ectopic pregnancy often occurs without any obvious risk factors.

These risk factors are divided into "high," "moderate," and "low" risk depending on the strength of the association with ectopic pregnancies. In other words, a "high" risk factor raises the risk of having an ectopic pregnancy much more than a "low" risk factor.

High Risk Factors for Ectopic Pregnancy

  • Previous ectopic pregnancy - Women who have had one ectopic pregnancy are around 17 times more likely to have an ectopic pregnancy than a woman who has not had an ectopic pregnancy.
  • 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; the majority of women who become pregnant today have not been exposed.
  • Endometriosis - Women who have endometriosis have an increased risk of ectopic pregnancy. Endometriosis can result in the formation of scar tissue and adhesions which may 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. Around a third of women who become pregnant following a tubal ligation will have an ectopic pregnancy.
  • Use of an IUD device - Contraceptive intrauterine devices (IUDs) have long been considered a risk factor for ectopic pregnancy, but researchers believe that IUDs do not technically increase the risk of ectopic pregnancy. Rather, if a conception occurs, the pregnancy has an increased risk of being ectopic. Overall, the risk of an ectopic pregnancy is four to times higher than someone who does not have an IUD, but this varies significantly depending on the type of IUD. This risk appears to be much higher if conception occurs while a Mirena is in place than with a Paragard.

    Moderate Risk Factors for Ectopic Pregnancy

    • History of sexually transmitted infections or pelvic inflammatory disease - Sexually transmitted diseases, some of which can lead to pelvic inflammatory disease, may result in scarring in the fallopian tubes, increasing the risk of an ectopic pregnancy. A history of PID is associated with a risk two to ten times higher than a woman who has not had PID.
    • History of infertility - Some medical factors that cause infertility might also make ectopic pregnancy more likely. In addition, it's thought that some of the drugs commonly used to treat infertility may increase the risk of an ectopic pregnancy.
    • 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 cigarette smoke, the higher the risk for ectopic pregnancy. In studies, women who smoke have a risk that is four times to 20 times higher than women who do not smoke.

    Low Risk Factors for Ectopic Pregnancy

    • 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 a few studies, women who had an appendectomy or laparatomy appeared to have a slightly increased risk of miscarriage.
    • Age - Risk of an ectopic pregnancy increases steadily with age, with moms over 40 having the highest risk.
    • Elective abortions - Women who have had two or more elective abortions may have a slightly increased risk of an ectopic pregnancy, though the research on this is inconclusive.

    Sources:

    Kashanian, M., Baradaran, H., Mousavi, S., Sheikhansari, N., and F. BararPour. Risk Factors in Ectopic Pregnancy and Differences Between Adults and Adolescents, Is Consanguinity Important?. Journal of Obstetrics and Gynecology. 2016. 36(7):935-939.

    Tulandi, T. Ectopic Pregnancy: Incidence, Risk Factors, and Pathology. UpToDate. Updated 04/13/2016.

    Zhang, D., Shi, W., Li, C. et al. Risk Factors for Recurrent Ectopic Pregnancy: A Case-Control Study. BJOG. 2016. 123 Suppl 3:82-89.

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