A new study has refined previous findings that hormone-related breast cancer risk factors influence the risk of breast cancer subtypes differently. The study included 1,197 breast cancer cases and 2,015 controls from the Los Angeles County and Detroit components of the Women's Contraceptive and Reproductive Experiences Study. Oral contraceptive use was found to be associated with a 2.9-fold increased risk of triple negative (ER-/PR-/HER2-) breast cancer among women of 45 to 64 years of age who had begun to use oral contraceptives before age 18.

Oral contraceptive use was not found to be associated with any other breast cancer subtype as defined by ER/PR/HER2/p53 status. Number of children was associated with a decreased risk of luminal A (ER+ or PR+/HER2-), luminal B (ER+ or PR+/HER2+), and ER-/PR-/HER2+ tumors. Older age at first full-term pregnancy was found to be associated with luminal A tumors among older women. Neither having children nor age at first birth was found to be associated with risk of triple-negative breast cancer. Breast-feeding for a long duration was found to be associated with lower risk of triple-negative and luminal A tumors. Risk patterns were not further influenced by p53 expression status.

Previous studies also linked birth control pills at young age and breast cancer

Previous studies have also found an association between birth control pill use at a young age and subsequent breast cancer, although not all studies are in agreement on this point. First regarded as an anomaly, then explained away as an unfortunate consequence of "early" formulations of the pill, the association has not always been regarded as important. However, for those of us with high risk young daughters who may yet make a decision as to whether to use the pill or similar types of birth control, it is worth paying attention to the findings.

Use of contraceptive injections and implants have not been widely studied. However, one study reported a marked increase in breast cancer risk among women using subdermal contraceptive implants compared to those using oral or injected contraceptives.