The collagenous stroma, glandular cells and ductal epithelium of the mammary gland, as well as the lumen they form, change with the 5 cycles of menstruation. 1. Under the influence of estrogen, on days 3 to 7 of the menstrual cycle, the epithelium proliferates to form 2 to 3 layers of B cells, making the lumen of the mammary cells inconspicuous. 2. During the follicular differentiation phase, which is the 8th to 14th day of the menstrual cycle, three types of epithelial cells can be distinguished. In the follicular phase, the stroma contains more collagen and this proliferative phase rarely has inflammatory cell infiltration. 3. On the 15th to 20th day of the menstrual cycle. With the onset of ovulation and the increase of progesterone and prolactin levels the breast tissue enters the luteal differentiation phase. This period increases the glycogen content in the mammary gland due to the progesterone hormone and is secreted into the stroma. The basal cells appear vacuoles and air sacs, but there is no hyperplasia, the basement becomes relaxed and the basement membrane becomes inconspicuous. 4. On days 21 to 27, the secretory phase occurs due to the influence of estrogen, progesterone and lactogen. Along with the appearance of progesterone, estrogen, insulin and glucocorticoids also increase, the stroma changes from dense to edematous, the tissue interstitial space is filled with fluid, and the veins become congested. During the menstrual period, from day 28 to day 2 of the menstrual cycle, progesterone and estrogen levels decrease, but the lumen remains swollen due to the secretion of eosinophilic granules, the basal cells still have air sacs containing glycogen, and the basement becomes dense and clearly demarcated again, with an increase in plasma cell infiltration. As seen from the above physiological changes in the breast, the breast becomes edematous and engorged on days 21 to 27 of the menstrual cycle, resulting in larger and tougher mammary glands with small nodules to the touch. Most women experience breast swelling, discomfort, or mild pain and tenderness during this period.