Urinary human chorionic gonadotropin among intrauterine device users: detection with a highly specific and sensitive assay.

The mechanism by which the intrauterine device (IUD) prevents clinical pregnancy has not been established. The possibility that the IUD works primarily at the time of implantation or beyond has raised legal and ethical concerns. Numerous studies have measured human chorionic gonadotropin (hCG) among IUD users as a way to determine whether implantation routinely occurs in the presence of an IUD. Both positive and negative findings have been reported, reflecting major difficulties in the measurement of hCG at low levels. In this study, we collected daily urine specimens for up to three menstrual cycles from 40 women who used nonmedicated IUDs. We assayed these urines with an immunoradiometric assay that is highly specific for hCG at very low concentrations. This assay allows us to describe patterns of hCG rise and fall that would be undetectable by standard radioimmunoassays. In this study of IUD users, we found only one incident of probable early pregnancy loss among 107 cycles. With the use of an assay sensitive to hCG at levels as low as physiologic background, the possibility of frequent unrecognized implantation among IUD users can be reasonably excluded.