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Different forms of estrogen and drugs that block estrogen have different effects on blood flow in the breast, according to a study in sheep. Estradiol-17beta, conjugated equine estrogen, and tibolone increased blood flow in the breast, whereas tamoxifen and raloxifene did not. Breast cancer is characterized by increased blood flow in the breast.

It appears many women experience premenstrual breast pain, often painful enough to seek medical attention. Questionnaires regarding premenstrual breast pain were completed by 1,171 women seen at an obstetrics and gynecology clinic. Overall, 69% of these women had experienced monthly premenstrual breast pain. Mammograms had been performed 4.7 times more frequently in young women with regular monthly breast pain compared to young women with no breast pain. Many women reported that their monthly breast pain significantly limited their everyday activities.

The proliferation of breast cells may vary according to the stage of the menstrual cycle. Researchers performed fine needle aspiration biopsy to obtain breast epithelial cells from 47 healthy, menstruating women without breast disease. More breast cells were found during the luteal phase of the menstrual cycle than during the follicular phase. Higher levels of progesterone were associated with greater proliferation of breast cells, indicating a stimulating effect of progesterone. Such findings may improve understanding of cancer risk and hormone treatment.

Estrogen receptors in breast cells appear to decline during the luteal phase of the menstrual cycle while the level of progesterone receptors appears to be the same during the follicular and luteal phases. Previous studies on cells from the endometrial lining of the uterus found that both estrogen and progesterone receptors decline in the luteal phase. Levels of estrogen and progesterone receptors in breast cells were measured in 42 healthy women using fine-needle aspiration biopsy. Estrogen receptor was detected in 68% of samples drawn during the follicular phase and in 32% of samples drawn during the luteal phase. However, progesterone receptor was detected in about 80% of the samples drawn during both phases.