The etiology of ectopic pregnancy is not well understood. However, several risk factors have been found to be associated with ectopic pregnancy (

Table 14-3).

A. Tubal Factors
Ectopic pregnancy is 5-10 times more common in women who have had salpingitis. In women with ectopic pregnancies, up to 50% will have had salpingitis previously and in most of these patients, the uninvolved tube is also abnormal. Other tubal factors that interfere with the progress of the fertilized ovum include adherent folds of tubal lumen due to salpingitis isthmica nodosa, developmental abnormalities of the tube or abnormal tubal anatomy due to DES exposure in utero, previous tubal surgery including tubal ligation with a 16-50% ectopic pregnancy rate if pregnancy occurs after tubal ligation, conservative treatment of an unruptured ectopic with a recurrent ectopic rate of 4-16%, and tubal anastomosis with a 4% ectopic rate. Adhesions from infection or previous abdominal surgery, endometriosis, and even leiomyomas have been associated with ectopic pregnancy. Most of these abnormalities are bilateral and irreversible.

B. Zygote Abnormalities
A variety of zygote abnormalities have been reported in ectopic pregnancy, including chromosomal abnormalities, gross malformations, and neural tube defects. The theory is that these abnormal preembryos are more likely to result in abnormal or ectopic implantation.

C. Ovarian Factors
Ovarian factors that may result in the development of an ectopic pregnancy are fertilization of an unextruded ovum, transmigration of the ovum into the contralateral tube with subsequent delayed and faulty implantation, and postmidcycle ovulation and fertilization.

D. Exogenous Hormones
Abnormal hormonal stimulation and/or exogenous hormones may play a role in ectopic gestation. For example, of pregnancies occurring in women taking progestin-only oral contraceptives, 4-6% are ectopic pregnancies. This may be due to progesterone’s smooth muscle relaxant effects and subsequent “ovum trapping.” Patients with DES exposure are also at risk, as are patients undergoing ovulation induction.

E. Other Factors
Intrauterine device (IUD) users are also at risk for ectopic pregnancy if pregnancy occurs, although the risk of ectopic pregnancy is still lower than if no contraceptive method is used. Whether the IUD prevents intrauterine but not ectopic pregnancy or whether an associated salpingitis is responsible for this increased risk is unclear. Smoking and increasing age are also associated with ectopic pregnancy. Multiple previous elective abortions are also felt to be a risk factor for ectopic pregnancy, as postabortal infection may lead to salpingitis.