Risk factors for ectopic pregnancy
This meta-analysis was undertaken with a view to introducing a screening programme for women in early pregnancy, at risk of ectopic pregnancy. It therefore aims to identify the risk factors.
The risk factors most strongly associated with ectopic pregnancy largely affect the fallopian tube, i.e. previous ectopic pregnancy, tubal surgery, in utero diethylstilbestrol exposure, tubal pathology and infertility. Previous genital infections and smoking are associated with a moderately increased risk.
WM Ankum et al. Risk factors for ectopic pregnancy: a meta-analysis. Fertility and Sterility 1996 65: 1093-1099.
The literature was searched using MEDLINE between 1978 and 1994 in English, French, German and Dutch. A manual search was undertaken of the 1993 and 1994 issues of the top ten gynaecological journals and the top five epidemiological journals of the Science Citation Index. Bibliographies of all identified papers were then reviewed. Case-control and cohort studies were examined. For case-control studies to be included cases had to be women with ectopic pregnancy confirmed by an operation report or by histopathological examination and controls had to be pregnant or non-pregnant women. Cohort studies had to compare women exposed to a risk factor with non-exposed women. All risk factors were examined, except contraceptive methods.
Twenty-seven case-control and nine cohort studies were identified. The number of participants in each study is not reported. If significant variability was found between results of individual studies they were not pooled.
Eighteen risk factors were identified. For each risk factor, the authors either report pooled odds ratios or the range of odds ratios (when results were not pooled) for case-control and cohort studies separately and within case-control studies, pregnant and non-pregnant comparison groups separately. Here, the results are summarised into categories according to strength of association.
Risk factors strongly associated with ectopic pregnancy
Previous ectopic pregnancy, tubal surgery and in utero diethylstilbestrol* exposure (*a synthetic form of oestrogen). (Odds ratios above 5). Studies investigating infertility and tubal pathology were not pooled. Results ranged from a moderate to a strong association.
Risk factors moderately associated with ectopic pregnancy
Previous genital infections (gonorrhoea, chlamydia, pelvic inflammatory disease) and a lifetime number of sexual partners exceeding one. (Odds ratios between 2 and 5). Results of studies investigating a history of, or current, smoking ranged from a slight to a moderate association.
Risk factors slightly associated with ectopic pregnancy
Previous pelvic and/or abdominal surgery, vaginal douching and having first sexual intercourse before 18 years old. (Odds ratios between 1 and 2). Studies investigating previous medical and spontaneous abortion were not pooled. Results ranged from no association to a slight one.
Given that the results were reported for three different groups and in two different ways, it was thought that categorising the risk factors would have more clarity and utility.
A thorough literature search was undertaken to include all risk factors. It is a shame that participant numbers were not reported and the majority of papers identified were case-control studies which are subject to various types of bias. Nevertheless, this paper is useful in identifying the risk factors for ectopic pregnancy and being able to say whether the risk is large or small.