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According to a study of identical twin pairs, the age a woman begins menstruating may influence her risk of
developing breast cancer, if her disease is caused by genes. However, researchers
know that not all breast cancer cases are caused exclusively by genes; some may be caused
by other factors too, such as hormones, diet, etc. In the study, the age that a woman
began menstruating was significant, if her cancer was hereditary. On the other hand, the
age of first menstruation was not important in women with non-hereditary breast cancer
(i.e., breast cancer thought not to be caused by genes). Instead, in these cases, other
factors, such as the number of children the women had, or the age at which they reached menopause, were more important in determining
their risk of breast cancer. According to researchers, this study helps show that
different types of breast cancer may be influenced by different risk factors.
Researchers know that hormones can be related to breast cancer. In fact, approximately
80% of breast cancers are dependent on the hormone estrogen in order to grow and survive.
Based on this knowledge, treatments such as the drug tamoxifen
have been developed to prevent estrogen from reaching breast cancer cells, thereby
"starving" and destroying these cells.
In the study, Ann S. Hamilton, PhD and Thomas M. Mack, MD, MPH of the University of
Southern California gathered information from 1,811 pairs of female twins, one or both of
whom had breast cancer. Because identical twins share the same genes, if both twins
developed breast cancer, the cancer was said to be hereditary (i.e., caused by genes); if
only one twin developed breast cancer, the cancer was said to be non-hereditary (or caused
by factors other than genes).
The researchers found that among the twins with hereditary breast cancers, the age that
they began menstruating was important in determining their risk of the disease. Women with
hereditary cancers were more likely to begin menstruating at an early age.
However, in the twin pairs in which only one twin developed breast cancer, the age of
first menstruation was not an important factor in disease risk. In other words, it did not
matter what age women with non-hereditary breast cancer began menstruating. Instead, among
these women, other factors, such as age at first childbirth, the number of children the
women had, and the age at which they reached menopause were more important in determining
their risk of breast cancer.
Researchers have long known that hormones play a key role in breast cancer, although
the nature of their role is still being investigated in studies such as this one.
Menstrual cycles and pregnancy both influence breast cancer
risk. It appears as though the more menstrual cycles a woman has over her lifetime, the
higher her risk of breast cancer. Thus, women who experience an early onset of menstruation (before age 12) or late menopause (after age 50) may be at a
higher-than-average risk of breast cancer.
Pregnancy also seems to decrease the risk of breast cancer if a woman becomes pregnant
with her first child before age 30. This is because there is an interruption of menstrual
cycles during pregnancy. Women who never become pregnant are at a higher risk of breast
cancer than those who have a first child before age 30. However, pregnancy after age 30
appears to also increase the risk of breast cancer. The older a woman is, the more likely
her breast tissue has already been exposed to some cancer-causing substances called
carcinogens. Therefore, exposure to elevated hormone levels during pregnancy at a later
age may stimulate the growth of abnormal breast tissue. In fact, some experts believe that
the number of menstrual cycles that occur between a woman’s first period and her
first pregnancy is a greater predictor of breast cancer risk than her age at her first
menstruation or menopause.
While it is too soon to conclusively say that hereditary breast cancers are influenced
by early menstruation while non-hereditary breast cancer are not, this study adds to a
body of research investigating the association between hormones and breast cancer.
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