What Causes Ectopic Pregnancy
Ectopic pregnancy results from two fundamental mechanisms: damage to the fallopian tube that impairs normal embryo transport, and factors that alter the quality of the fertilized ovum or hormonal environment. 1, 2
Primary Causative Mechanisms
Tubal Damage and Transport Dysfunction
The integrity of the fallopian tube is the most critical factor determining ectopic pregnancy risk. 2
Highest risk conditions include:
- Tubal surgery (including sterilization procedures) represents the single highest risk category for developing ectopic pregnancy 1
- Salpingitis/pelvic inflammatory disease (PID) doubles the risk with each recurrent episode due to progressive tubal occlusion and scarring 1
- Previous ectopic pregnancy creates significant risk increase, either from surgical management effects or persistence of the original underlying risk factors 1
- Intrauterine devices can alter tubal transport mechanisms 2
- Salpingitis isthmica nodosa causes structural tubal abnormalities 2
- DES exposure affects tubal anatomy and function 2
Altered Ovum Quality and Hormonal Environment
Factors that theoretically alter the fertilized ovum or hormonal milieu contribute to ectopic implantation. 2
Key contributors include:
- Ovulation induction may alter the normal hormonal environment 2
- Delayed ovulation can affect embryo-endometrial synchrony 2
- Transperitoneal ovum migration increases the distance and time for tubal transport 2
Assisted Reproductive Technology as a Modern Risk Factor
In vitro fertilization (IVF) and induced ovulation have become increasingly important causative factors in contemporary practice. 1 However, even with IVF, the underlying tubal factors (previous salpingitis, tubal surgery, prior ectopic pregnancy) remain the most important determinants of risk. 1
Heterotopic pregnancy (simultaneous intrauterine and ectopic) increases with greater numbers of implanted embryos and ovarian stimulation. 1 Notably, no relationship exists between spontaneous versus stimulated ovulation, type of ovarian stimulation, or number of implanted embryos and ectopic pregnancy risk in isolation. 1
Clinical Context and Epidemiology
Approximately 1 in 100 pregnancies are ectopic, with the conceptus usually implanting in the fallopian tube. 3 In the United States, the estimated prevalence is 1% to 2%, and ruptured ectopic pregnancy accounts for 2.7% of pregnancy-related deaths. 4 Ectopic pregnancy accounts for 9% of all pregnancy-related maternal deaths in the United States and remains the most common cause of maternal death and serious morbidity in the first trimester. 1
Important Clinical Pitfalls
Do not assume that absence of classic risk factors excludes ectopic pregnancy—while tubal damage represents the primary mechanism, ectopic pregnancy can occur in women without identifiable risk factors. 4, 5
Risk factors may be cumulative, particularly when combining previous ectopic pregnancy or tubal surgery with pelvic post-inflammatory status or presence of an intrauterine device. 6