Abstract
Background. There are several methodological problems when studying contraceptive efficacy. The present study focuses on efficacy issues from the use of oral contraceptives (OC). Methods. Through a computerized system of medical records kept at the Department of Gynecology, Regional Hospital of Trondheim, Norway, 800 OC-failure pregnancies were identified among 8192 women having pregnancy termination from January 1, 1987 to December 31, 1996. Ascertainment of cases was done by validating hospital data on OC failures by data collected by the referring practitioner. Inconsistencies in failure data were found among 257 of the 800 (32%) women who claimed at the hospital that an OC was in use at time for conception. The medical record kept by their practitioners stated that 158 women had stopped using OC before last menstrual period, of the remaining 99 women neither data on actual use nor data on prescription could be confirmed. Results. Over the 10-year study period the proportion of failures by brand of OC was remarkably constant from year to year and reflected the user pattern of OCs in Norway. One-third of the total number of failures ( n =523) were claimed to be method failures. The annual overall OC-failure rate was estimated to be 1% during the 10-year study. Conclusion. Case-control studies focusing on efficacy of OCs must be carefully designed and comprise both women carrying a pregnancy to term as well as women terminating their pregnancies. Our study focused on only one segment of the population of women experiencing an OC failure: women having pregnancy termination. The present study has shown that exposure data collected from interviews have to be validated against prescription data.