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Figure 1: Human breast lobules at different stages of development. Cancers are known to arise in the immature breast tissue that all girls are born with - Type 1 lobules (left panel) are TDLU's, known as "terminal ductal lobular units." Some Type 1 lobules mature at puberty into Type 2 lobules. Scientists recognize Types 1 and 2 lobules as cancer-vulnerable. They recognize Type 3 lobules (right panel) as cancer-resistant.* Notice the considerable difference in breast tissue maturity between Types 1 and 3 lobules.
During a normal pregnancy, women are overexposed to the hormone estrogen (a recognized carcinogen). Estrogen causes breast tissue to grow. It stimulates Types 1 and 2 lobules to multiply. Abortion leaves a woman with more Types 1 and 2 lobules.
Only a third trimester process in pregnancy protects a woman from estrogen overexposure and matures her breast tissue into cancer-resistant, milk producing breast tissue (Types 3 & 4 lobules). A full term pregnancy leaves a woman with an increase in cancer-resistant breast lobules. That's why women with more children have a lower lifetime risk for the disease.
A woman is the most vulnerable to carcinogens between the time that her menstrual periods begin and her first full term pregnancy occurs. With every menstrual period, estrogen stimulates Types 1 and 2 lobules.
*Whole mount preparation, toluidine blue stain (Lob 1, 2 and 3: x99, x118 and x121, respectively).
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