IUDs protect against intrauterine pregnancies better than against ectopic pregnancies. Thus, when an IUD user becomes pregnant, the pregnancy is more likely to be ectopic than a pregnancy in another woman (369, 424, 442, 463). In IUD users an estimated one in 25 to 30 pregnancies, or 3% to 4%, is ectopic. Rates of ectopic pregnancy in the general population vary. In the US, Canada, and Europe, more than 1% of all pregnancies are ectopic (543, 628).

Any pregnancy in an IUD user is uncommon, however. Thus ectopic pregnancy in an IUD user is rare. For most of the widely used IUDs, less than 1.5 ectopic pregnancies occur per 1,000 woman-years of IUD use (69, 73, 314, 327, 330, 335, 543).

Differences among devices. With inert and copper-bearing IUDs, ectopic pregnancy rates seem to reflect overall effectiveness. For example, two of the most effective IUDs, the TCu-380A and MLCu-375, have the lowest ectopic pregnancy rates—0.25 and close to 0 per 1,000 woman-years, respectively, according to results of major clinical trials (69, 73, 314, 330, 335, 543, 648). The highly effective LNG-20 IUD has had even lower ectopic pregnancy rates than most copper IUDs with which it has been compared (546, 556), perhaps because it sometimes suppresses ovulation (257). In contrast, the Progestasert has had higher rates—4 to 5 per 1,000 woman-years (116, 327, 400).

While some questions about IUDs and ectopic pregnancy remain unanswered (see Ectopic pregnancy and past IUD use in Chapter 5.5), the clinical implications are clear:

• Women using IUDs should be told about the signs of ectopic pregnancy. They should know the signs abdominal pain, dark and scanty or intermenstrual bleeding along with the usual signs of pregnancy and to return for care promptly if they appear.

• If an IUD user conceives or shows signs of pregnancy, health care providers should always look for ectopic pregnancy.