© 1989 by Oxford University Press
Journal of the National Cancer Institute, Vol. 81, No. 13, 1000-1004,
July 5, 1989
© 1989 Oxford University Press
Early Oral Contraceptive Use and Breast Cancer Among Premenopausal Women: Final Report From a Study in Southern Sweden
Department of Oncology, University Hospital Lund, Sweden
Southern Swedish Regional Tumor Registry Lund
Department of Community Health Sciences, Lund University, Malmö General Hospital Malmö, Sweden
*Correspondence to: Dr. H. Olsson, Department of Oncology, University Hospital, Lund, S-221 85 Sweden
In southern Sweden during the 1960s, women began to use oral contraceptives (OCs) extensively at a young age. This case-control study investigates the relationship between the use of OCs and breast cancer development in women in southern Sweden diagnosed in the early 1980s. The risk for breast cancer after OC use among premenopausal women was modeled, after adjustment was made for age, age at menarche, and age at first full-term pregnancy or parity. Both the duration of OC use before 25 years of age and commencement of OC use at a young age were associated with a significant increase in the risk of breast cancer as well as a significant trend. The duration of OC use before the first full-term pregnancy was associated with an increased risk of breast cancer, but it did not show a significant trend. The total duration of OC use was wealdy, but not significantly, associated with breast cancer development. The odds ratio for women starting OC use before 20 years of age was 5.8 [95% confidence interval (CI), 2.6–12.8]; for women using OCs for >5 years before age 25, it was 5.3 (95% CI, 2.1–13.2); and for women using OCs for 
8 years before first full-term pregnancy, it was 2.0 (95% CI, 0.8–4.7). In multivariate analyses including the different measurements of OC use, only starting age of OC use was significantly associated with breast cancer. The exposure-response relationship between duration of OC use and risk of breast cancer depended on the age at first use of OCs. Given a fixed duration of OC use, the risk increased with younger starting age of OC use. The findings point to the importance of the early reproductive years as risk determinants for breast cancer after OC use.
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