Does My Age at First Pregnancy Affect My Risk for Breast Cancer?

— -- Question: Does the age at which I had my first pregnancy affect my risk for breast cancer?

Answer:It's interesting that a woman who has her first full-term pregnancy by the age of 18 has about one-third the risk of developing breast cancer as a woman who has her first full-term pregnancy after age 30. We don't know exactly why that is but the data seems to indicate that it's the breast while its developing that is most susceptible to environmental factors -- carcinogens, cancer-causing agents in the environment that may trigger a cancer later on in life, even

While the breast is developing we think there is a lot of cells that haven't been defined as how they're going to be, in other words, they haven't become breast cells yet. They're what we call undifferentiated; they haven't completed their development into true breast cells. That makes them more susceptible to cancer-causing agents, to carcinogens.

So if the breast takes a long time to develop, and the usual female menarche -- the beginning of periods starts now around age 11 or 12 -- we don't know when the breast is completely developed but it may take many years for all of this branching and differentiation to occur. That means there is a long time over which exposure to a carcinogen could in fact cause damages to the DNA and could become a cancer.

The breast develops very quickly when a full-term pregnancy occurs because the breast has to get ready to start making milk, so that terminal differentiation, as we call it -- the cells of the lobule becoming lobular cells that are pretty much fixed and won't become cancer cells -- takes place very quickly if a full-term pregnancy occurs early in life. And we think that that reduces the window of opportunity for a carcinogen to cause those cells to become cancer cells, whereas the woman who doesn't have a full-term pregnancy for many, many years probably has more cells that can be susceptible to carcinogens.

So full-term pregnancy early in life seems to offer some protection. It's not perfect and it's certainly not a good reason to get pregnant, but it does offer some protection from breast cancer.

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