Spontaneous abortion is the most frequent complication of pregnancy with an IUD in place. Some 50% to 60% of uterine pregnancies spontaneously abort if the IUD is not removed (4, 195, 358, 371, 413, 463). This is 21/2 to 5 times more often than in other women (186, 413, 459). More than half of the spontaneous abortions in IUD users occur in the second trimester (215, 358).Early studies found that septic (infected) second-trimester spontaneous abortion—a rare but life-threatening event—was more common in women whose IUDs were left in place than in women not using IUDs at the time of conception (109). This increase in risk was associated, however, with the Dalkon Shield IUD, which is no longer available, and there is no conclusive evidence linking other types of IUDs with increased risks of septic abortion (488).

Because infection can occur in any pregnancy with an IUD in place, the IUD should be removed as soon as pregnancy is confirmed (1, 437). This virtually eliminates any risk of septic abortion and reduces risk of spontaneous abortion to the same levels faced by other women (4, 109, 195, 371, 557).

An IUD left in place during pregnancy also increases the risk of premature delivery (413, 463, 488). It does not increase the risk of other complications—birth defects, genetic abnormalities, or molar pregnancy (a uterine growth that mimics pregnancy) (152, 232, 276, 344, 371, 413, 488, 557).