Editor—In their review of ectopic pregnancy, Tay et al report that previous female sterilisation and current use of an intrauterine contraceptive device are risk factors only when patients with ectopic pregnancy are compared with pregnant controls and not with non-pregnant women.1 However, we published a review on the effects of oral contraceptives after fertilisation, in which we reviewed data indicating that the ratio of extrauterine to intrauterine pregnancies is increased for women taking combined oral contraceptives and progestogen only pills, compared with control groups of pregnant women not using oral contraceptives.2
The increased odds ratio of ectopic pregnancy in women taking the combined pill (compared with pregnant controls) was found to be 4.5 (95% confidence interval 2.1 to 9.6)3 and 13.9 (1.8 to 108.3)4 in studies including a total of 484 women with ectopic pregnancies and 289 pregnant controls. We used these odds ratios to estimate that in the studied populations the absolute rate of ectopic pregnancy in women using combined pills would range from 0.7 to 19.9 ectopic pregnancies per 1000 woman years. We could, however, find only one study, from Zimbabwe, that reported an absolute risk of ectopic pregnancy in women taking the combined pill—an ectopic rate of 0.5 per 1000 women years.
The increased odds ratio of ectopic pregnancy for a woman taking progestogen only pills (compared with pregnant controls) was reported in one study to be 79.1 (8.5 to 735.1),5 which we used to estimate an absolute risk of 4-79 ectopic pregnancies per 1000 woman years. This prediction is reasonably concordant with the reported absolute rates of ectopic pregnancy in women taking progestogen-only pills of 3-20 per 1000 woman years.
Like Tay et al, and most researchers in this field, we restricted our review to studies using pregnant controls because when considering the situation where a woman became pregnant during the use of a birth control agent one should focus on pregnant controls.
We believe that the association of ectopic pregnancies with oral contraceptives has been overlooked in the medical literature on ectopic pregnancy and that most who prescribe or dispense oral contraceptives are not aware of this association. If a woman who is taking an oral contraceptive presents with pelvic pain and unusual vaginal bleeding, we would recommend that the possibility of ectopic pregnancy be ruled out by using the wise and practical clinical approach suggested by Tay et al.
Footnotes
Competing interests: None declared.
References
- 1.Tay JI, Moore J, Walker JJ. Ectopic pregnancy. BMJ. 2000;320:916–919. doi: 10.1136/bmj.320.7239.916. . (1 April.) [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Larimore WL, Stanford JB. Postfertilization effects of oral contraceptives and their relationship to informed consent. Arch Fam Med. 2000;9(2):126–133. doi: 10.1001/archfami.9.2.126. [DOI] [PubMed] [Google Scholar]
- 3.Coste J, Job-Spira N, Fernandez H, Papiernik E, Spira A. Risk factors for ectopic pregnancy: a case-control study in France, with special focus on infectious factors. Am J Epidemiol. 1991;133:839–849. doi: 10.1093/oxfordjournals.aje.a115964. [DOI] [PubMed] [Google Scholar]
- 4.Thorburn J, Berntsson C, Philipson M, Lindbolm B. Background factors of ectopic pregnancy. I. Frequency distribution in a case-control study. Eur J Obstet Gynecol Reprod Biol. 1986;23:321–331. doi: 10.1016/0028-2243(86)90167-x. [DOI] [PubMed] [Google Scholar]
- 5.Liukko P, Erkkola R, Laakso L. Ectopic pregnancies during use of low-dose progestogens for oral contraception. Contraception. 1977;16:575–580. doi: 10.1016/0010-7824(77)90058-0. [DOI] [PubMed] [Google Scholar]