Case-control study of risk factors for cervical squamous-cell neoplasia in Denmark. III. Role of oral contraceptive use.

The role of oral contraceptive (OC) use in relation to the risk of cervical neoplasia (squamous cell) was investigated in a population-based case-control study in Denmark of 586 women with histologically verified cervical carcinoma in situ (CIS), 59 women with invasive cervical cancer from Copenhagen, and 614 controls drawn at random from the female population in the study area. Ever use of OCs was associated with an increased crude risk for carcinoma in situ (relative risk [RR] = 1.8, 95 percent confidence interval [CI] = 1.3-2.5). The crude risk in relation to invasive carcinoma was 1.6 (CI = 0.7-3.5). After adjustment for potential confounders (excluding human papillomavirus), the risks were marginally increased, but not statistically significant (CIS: RR = 1.4, CI = 0.9-21: invasive: RR = 1.3, CI = 0.5-3.3). The risk increased with duration of use; compared with never users, the adjusted RR for carcinoma in situ was 1.9 (CI = 1.1-3.1) for women who had used OCs for six to nine years, and 1.7 (CI = 1.0-2.7) for women who used OCs for 10 years or more. This was independent of years since last use since both recent and non-recent long-term users were at an increased risk. This trend in risk with duration did not apply to the same extent to invasive lesions. The observation that the risks related to OC use were found both in women who had ever had a Pap smear and in women who had never been screened previously may speak against detection bias as an important factor.