Hormone contraception before the first birth and endometrial cancer risk.

There is a well-documented reduction in endometrial cancer risk with combined oral contraceptive (COC) use. COC use before the first full-term pregnancy may affect breast cancer risk for decades, but this relationship has not been investigated in endometrial cancer. We investigated the risk for endometrial cancer with COC use before the first full-term pregnancy. Cases (n = 524) from a population-based cancer registry and age-matched controls (n = 1,032) were recruited between 2002 and 2006 in Alberta, Canada. Participants completed an in-person interview and provided detailed information on exogenous hormone use and other risk factors. Risk reductions in endometrial cancer with COC use over the premenopausal years were consistent with the published literature. We also found evidence of a long-term, significant risk reduction in parous women with COC use before the first full-term pregnancy. Among parous women, ≥5 years of COC use before a first full-term pregnancy was associated with a significant reduction in risk [adjusted OR, 0.42; 95% confidence interval (CI), 0.25-0.72], even if this exposure was a woman's only use of COCs (adjusted OR, 0.35; 95% CI, 0.18-0.68). Further understanding of the long-term effects of COC use may help guide the timing of chemoprevention efforts via COCs.