Type 1 lobules, the most primitive and undifferentiated, are present in small girls. After first menstruation, some breast tissue develops into Type 2 lobules. These are more complex and include more ductules per lobule.
Type 1 and 2 lobules are where ductal cancers develop. The most cancer-resistant tissue, types 3 and 4 breast lobules, only develops during the third trimester of pregnancy.
Due to early pregnancy growth spurt during the first two trimesters, the woman who has an induced abortion is left with way more Types 1 and 2 lobules than she had before her pregnancy began. This leaves her with more places for cancer to start. By contrast, the woman who has a full term pregnancy is left with more mature, cancer-resistant Types 3 and 4 lobules than she had before her pregnancy began. This results in the protective effect of a full term pregnancy.
If her first pregnancy is full term, this will have a strong protective effect. Subsequent full-term pregnancies will have weaker, but still measurable protective effects.
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