Breast Cancer: Hormonal Risk Factors

It is not yet fully understood what the hormonal risk factors are, but there have been some interesting clues. It is known that it has something to do with age and the menstrual cycle: the younger the woman is at her first period and the older she is when she goes to menopause, the more likely she is to get breast cancer. It seems that the longer a woman has reproductive levels of hormones, the more susceptible she is to breast cancer. If she menstruates at more than 40 years of age, she seems to have a particularly high risk. If the ovaries are removed early and no hormone replacement is given, the risk of getting breast cancer is greatly reduced. It is not exactly an all-cure, sadly, since it would also heighten the danger of osteoporosis. If a woman has had a hysterectomy (removal of the ovaries), it may or may not influence her vulnerability to breast cancer, depending upon whether the ovaries, as well as the uterus, are removed. If a woman still has her ovaries, her body is still going through hormonal cycles, even though there aren't any more periods.
Pregnancy also seems to affects breast cancer risk. Women who have never been pregnant appear to be more at risk than women who have had children before the age of 30 and women who have their first pregnancies after 30 have a greater risk than women who have never been pregnant at all. The hormones of pregnancy carried to term will mature the breast tissue in a young woman. The same hormones after 30 may actually stimulate breast tissue that has already been mutated. Several studies indicate that a pregnancy that ends in miscarriage or abortion slightly increases breast cancer risk.
The key seems to be the amount of time between the first period and the first pregnancy. There is a lot of speculation as to why this is so. One possible explanation is that between menarche and the first pregnancy the breast tissue is especially sensitive to carcinogens (cancer promoting agents). This seems to be true. Several factors such as diet, alcohol consumption and radiation exposure all seem to have a greater effect on a woman's breasts between her first period and her first pregnancy than they do later. Thus, the developing breast is more susceptible to carcinogens than the breast that has gone through its complete hormonal development. The increased vulnerability may relate to the cells' capability of mutating up until the first pregnancy. There seems to be something about the first pregnancy of a woman that halts the cells from being able to sustain a mutation, the greater the chance that they'll mutate in response to a carcinogen and in a way that develops into breast cancer.
A factor relating to the number of menstrual cycles is breast-feeding. Recent findings have shown that women who breast-feed for a long period of time, more than 6 consecutive years, have a decreased risk of breast cancer. In addition, women who have had early pregnancies and have breast-fed have a diminished risk of subsequent breast cancer. This is most likely related to fewer ovulatory cycles at a crucial moment in the reproductive life.
It is now obvious that most studies on hormonal risk factors of breast cancer are still very much at the theorizing stage: as yet, it is not known why there is this vulnerable time in a woman's life and why or how internal hormones affect breast cancer.