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Factors that allow prolonged unopposed lifetime effect of estrogen on the breast

Please note that this is the single most important factor which is directly responsible for breast cancer, and explains why there has been a surge in the numbers of cases with breast cancer.
Breast tissue is responsive to the hormone estrogen, and prolonged action of estrogen on the breast is not desirable; in a majority of breast cancers, estrogen is responsible to 'drive' the cancer cells. The following image represents hormonal changes in a normal menstrual cycle:

In a normal menstrual cycle of 28 days, the predominant hormone in the first 14 days is estrogen. On around the 14th day, ovulation occurs (follicle is released from ovary), and then the other hormone 'progesterone' takes over for the next 14 days, and levels of estrogen goes down. Majority of gynecological disorders and diseases affect only the first half of this menstrual cycle and the second half remains unaltered. So if a lady has a prolonged periods for whatever reason, it is always the first half which is prolonged and the second half stays remarkably constant (14 days). The foll image represents a prolonged menstrual cycle:
For example, a lady has periods lasting 35 or 40 days each (due to, say, Polycystic Ovarian Syndrome), the first half will be 21 or 26 days respectively, but the second half will be constant (14 days). So here, estrogen has had more time to act on the breast. One cycle like this doesn't make a difference, but long standing disorders do.

So with this fact in mind that prolonged unoppossed effect of estrogen on the breast tissue is undesirable, let us enumerate what all factors will allow prolonged, unopposed action of estrogen on the breast:

  • Menstrual History: Early menarche (the age when the first menstrual cycle started), and late menopause (the last menstrual cycle) are both responsible for an increased risk of developing breast cancer, the principle being the same, estrogen having more time to act on the breast. Over decades, the average age of menarche is shifting to early due to a variety of causes.

  • Reproductive History: A lady who has the first child after 30 years of age has an increased risk of developing breast cancer. And so is the risk for a lday who has never borne a child. Pregnancy halts the action of estrogen; progesterone is the main hormone of pregnancy. Pregnancy has a protective effect against breast cancer. However, in a very small section of ladies, who are seen to develop breast cancer within a few years of delivery, it has been postulated that the surge (spurt) of estrogen that occurs in very early pregnancy, may actually be responsible, and then the pregnancy has no protective effect here. Hence we do see a few cases of breast cancer during pregnancy or within a few years of delivery. These tend to be very aggressive cancers

  • History of breast cancer: A previous history of breast cancer increases the chances of developing a cancer in the other breast

  • Disorders affecting menstrual cycles: It has been postulated that disorders which prolong the menstrual cycles may cause an increased risk of developing breast cancer. There are no studies, which have proven this.

A combination of the factors above may increase the risk further. For example, an early menarche (say 11 years of age), and a late first pregnancy (beyond 30 years of age) will allow estrogen to act on the breast for a significant number of years (say 19 years in this case), and increase the risk of breast cancer. On the other hand, a later menarche (say 14 years of age), and not so late pregnancy (say 26 years of age), may give estrogen about 10 yrs to act, and such a lady may not have an increased risk of breast cancer.

To note, our understanding of breast cancer is only a tip of the iceberg, and much more needs to be known. What has been stated above is only what we have been able to understand so far and is not mathematics, so it is not that all ladies with increased risk will develop breast cancer, and all with non increased risk will not. Any of them may develop a cancer. The importance of understanding these risk factors is to be alert and detect cancer at an early stage.