The Estrogen Factor

Pregnancy: The Strongest Protective Factor

Pregnancy provides one of the strongest protective factors from breast cancer. Medical experts have universally recognized since the publication of a landmark Harvard study in 1970 that the earlier a woman has her first full term pregnancy, the lower is her risk for breast cancer. The study estimated that "women having their first child when under age 18 years have only one third the breast cancer risk of those whose first birth is delayed until age 35 years or more."

Breastfeeding has also been considered a likely protective factor, but this wasn’t confirmed until recently. In July 2002, a large meta-analysis of 47 epidemiological studies conducted in 30 countries and published in the British medical journal, Lancet, determined that women can reduce their relative risk of the disease by 4.3% for every 12 months of breastfeeding and 7.0% for each birth. It was concluded that skyrocketing breast cancer rates in the developed nations could be reduced by more than one-half if only women would carry through with their pregnancies.

Estrogen Overexposure

Most of the risk factors associated with breast cancer involve estrogen overexposure. Estrogen is a secondary carcinogen. It promotes the growth of normal and abnormal tissue. According to the Canadian Breast Cancer Network, "Natural estrogens are thought to be a major factor in breast cancer risk. During the menstrual cycle, the ovaries produce a potent form of estrogen called estradiol. The greater someone's lifetime exposure to estradiol (the more menstrual cycles from puberty to menopause), the higher the risk of developing breast cancer."

So, women who reach puberty at an early age or menopause at a late age or who have fewer or no children, experience more menstrual cycles. Therefore, they are known to have a higher risk of breast cancer. Women who have more children and who nurse them, on the other hand, experience fewer menstrual cycles and reduce their risk of breast cancer by doing so. Similarly, factors such as avoidance of alcohol can reduce a woman’s exposure to estrogen.

A never-pregnant woman has a network of primitive, immature and cancer-vulnerable breast cells which make up her milk glands. When she becomes pregnant, her breasts enlarge. This occurs because a surge of estradiol in the blood reaches twenty fold, or a 2,000% increase, in the first trimester causing an explosive growth of breast tissue. This process is called "proliferation" and is the time when breast cells are most vulnerable to carcinogens.

If the pregnancy is carried to term, a second process called "differentiation" propels these newly produced breast cells into a maturing phase that greatly reduces the risk of future breast cancer. Differentiation is the shaping of cells into milk producing tissue. It shuts off the cell multiplication process and occurs at approximately 32 weeks gestation.

If the pregnancy is aborted, the woman is left with more undifferentiated -- and therefore dangerously stimulated cancer-vulnerable cells -- than she had before she was pregnant. On the other hand, a full term pregnancy leaves a woman with more milk producing differentiated cells, which means that she has fewer cancer-vulnerable cells in her breasts than she did before the pregnancy.

Research has shown that most miscarriages do not raise breast cancer risk. This is due to a lack of estrogen overexposure. Miscarriages are frequently precipitated by a decline in the production of progesterone, which is needed to maintain a pregnancy. Estrogen is made from progesterone, so the levels of each hormone rise and fall together during pregnancy. If a miscarriage is caused by a decline in the level of progesterone, than the decline would be seen in the level of estrogen as well.