Risk factors
Although women with ectopic pregnancy frequently have no identifiable risk factors, a prospective case-controlled study has shown that increased awareness of ectopic pregnancy and a knowledge of the associated risk factors helps identify women at higher risk in order to facilitate early and more accurate diagnosis.Most risk factors are associated with risks of prior damage to the Fallopian tube (Box 1). These factors include any previous pelvic or abdominal surgery, and pelvic infection. Chlamydia trachomatis has been linked to 30-50% of all ectopic pregnancies. The exact mechanism of this association is not known but it has been proposed that in addition to distortion of tubal architecture, it may to be due to an effect on the tubal microenvironment. Ectopic pregnancy is more common in women attending infertility clinics even in the absence of tubal disease. In addition, the use of ART increases the rate of ectopic pregnancies. In vitro fertilisation (IVF) is associated with an ectopic pregnancy risk of 2-5% and it may be higher than this where there is tubal disease. Indeed the first IVF pregnancy, before the first IVF live birth, was a tubal ectopic pregnancy. Some types of contraception, such as progestogen-only contraception and the intrauterine contraceptive device, are associated with an increased incidence of ectopic pregnancy when there is contraceptive failure, without necessarily increasing the absolute risk of ectopic pregnancy. One third of all cases of ectopic pregnancy are thought to be associated with smoking. There is a dose–effect relationship, with the highest adjusted odds ratio (OR) (3.9) when more than 20 cigarettes are smoked a day. Several mechanisms for this association have been suggested, including one or more of the following: delayed ovulation, altered tubal and uterine motility and microenvironment, or altered immunity.1920 The risk of ectopic pregnancy increases with advancing maternal age, with age over 35 years being a significant risk factor.6 Hypotheses for this association include the higher probability of exposure to most other risk factors with advancing age, increase in chromosomal abnormalities in trophoblastic tissue and age-related changes in tubal function delaying ovum transport, resulting in tubal implantation. Women with a previous history of ectopic pregnancy also have an increased risk, which increases further in proportion to the number of previous ectopic pregnancies. In one study the OR for having an ectopic pregnancy was 12.5 after one previous ectopic pregnancy and 76.6 after two.