The risk of pelvic inflammatory disease (PID) associated with use of an intrauterine device (IUD) has been an important concern that has dominated decisions on its use throughout the world, especially in the USA. Early research that suggested such an association led to both a dramatic decline in use of the method and its withdrawal from the US market by two manufacturers. However, factors other than use of an IUD are now thought to be major determinants of PID risk. To address these concerns, we have reviewed the World Health Organisation's IUD clinical trial data to explore the incidence and patterns of PID risk with use of an IUD. The overall rate of PID among 22 908 IUD insertions and during 51 399 woman-years of follow-up was 1.6 cases per 1 000 woman-years of use. After adjustment for confounding factors, PID risk was more than six times higher during the 20 days after insertion than during later times (unadjusted rates, 9.7 vs 1.4 per 1 000 woman-years, respectively); the risk was low and constant for up to eight years of follow-up. Rates varied according to geographical area (highest in Africa and lowest in China) and were inversely associated with age. PID rates were lower among women who had IUDs inserted more recently. Our findings indicate that PID among IUD users is most strongly related to the insertion process and to background risk of sexually transmissible disease. PID is an infrequent event beyond the first 20 days after insertion. Because of this increased risk with insertion, IUDs should be left in place up to their maximum lifespan and should not routinely be replaced earlier, provided there are no contraindications to continued use and the woman wishes to continue with the device.
