The intrauterine device and pelvic inflammatory disease: the Women's Health Study reanalyzed

The Women's Health Study (WHS) was a large, widely accepted and influential case-control study of the relationship between the use of intrauterine contraceptive devices (IUDs) and pelvic inflammatory disease (PID). The data were collected at 16 hospitals in 9 cities across the U.S.A. from October 1976 through August 1978. The first paper on this research was published in 1981 and concluded that IUDs increase the risk of PID. The report cited an estimated RR (relative risk) of PID for current IUD users vs nonIUD users of 1.6 with a 95% confidence interval of (1.4, 1.9). However, careful examination of the report reveals that the data support conclusions antithetical to those at which the author arrived. When the second report on the WHS was published in 1983, it was anticipated that many of the shortcomings of the first report would be corrected, but they were not. In 1983 we undertook a complete reanalysis of the same WHS data using more appropriate criteria and the results were compared to the first two published reports. The reanalysis revealed an RR of 1.02 (0.86, 1.21) for current IUD users compared to noncontraceptors. The conclusion of the WHS should have been that IUDs do not increase the risk of PID.