The question the reader will ask at this point is ‘Given all this epidemiological study, do we know the causes of cancer?’ Broadly the answer is ‘yes’ in many circumstances and for many cancers, and the opportunities for prevention that this understanding generates are there to be taken. We do not always know how the factors that have been identified by the epidemiological studies discussed in this chapter link up to what is being learned in the laboratories of the molecular biologists. This connection is being made rapidly and will be increasingly clear by the end of the century. Epidemiology has been very successful in discovering or confirming which features of our lives in the Western world can be now identified as causes of cancer.
A few hormones can certainly cause cancer. When large quantities of the female hormone oestrogen were given for medical reasons during pregnancy, rare kinds of cancer were liter found in the daughters of these women. Large quantities of oestrogens which were formerly given to women for menopausal symptoms undoubtedly caused cancer in the uterus although newer preparations do not. Oral contraceptives represent a complicated case, perhaps involving a real increased risk of breast cancer, particularly in young people. On the other hand, the Pill is capable of reducing the risk of ovarian cancer and cancer of the body of the uterus. Both risks and benefits can therefore be claimed for the Pill and controversy about its use is likely to continue.
At a more subtle level, it is possible that some of the effects of child-bearing may include a reduction in some cancer risks, probably generated by changing hormone levels. This is particularly apparent in cancer of the ovary, where having been pregnant appears to be protective.
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