Meditations of an oncology geek

Can pregnancy prevent breast cancer?

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16 October 2011

Commentary on original research

In the spirit of breast cancer awareness month, I thought I’d make my readers more aware of a phenomenon that has a very pronounced influence on a woman’s lifetime breast cancer risk. This phenomenon could account for up to a third of breast cancer risk factors. To put that in perspective, genes and genetically inherited breast cancer risk accounts for at most 10% of all breast cancer risk in a population. This phenomenon I am about to discuss is currently, in my opinion, the most plausible mechanism for the increased rates of breast cancer in first world countries as well.

Here it is: pregnancy. (1)

Younger age of first tull-term pregnancy and number of pregnancies are strongly associated with a reduction in lifetime risk of breast cancer.

The article’s abstract is included below:

INTRODUCTION:Although pregnancy-related factors such as nulliparity and late age at first full-term pregnancy are well-established risk factors for invasive breast cancer, the roles of these factors in the natural history of breast cancer development remain unclear.
Among 52,464 postmenopausal women participating in the California Teachers Study (CTS), 624 were diagnosed with breast carcinoma in situ (CIS) and 2,828 with invasive breast cancer between 1995 and 2007. Multivariable Cox proportional hazards regression methods were used to estimate relative risks associated with parity, age at first full-term pregnancy, breastfeeding, nausea or vomiting during pregnancy, and preeclampsia.
Compared with never-pregnant women, an increasing number of full-term pregnancies was associated with greater risk reduction for both breast CIS and invasive breast cancer (both P trend < 0.01). Women having four or more full-term pregnancies had a 31% lower breast CIS risk (RR = 0.69, 95% CI = 0.51 to 0.93) and 18% lower invasive breast cancer risk (RR = 0.82, 95% CI = 0.72 to 0.94). Parous women whose first full-term pregnancy occurred at age 35 years or later had a 118% greater risk for breast CIS (RR = 2.18, 95% CI = 1.36 to 3.49) and 27% greater risk for invasive breast cancer (RR = 1.27, 95% CI = 0.99 to 1.65) than those whose first full-term pregnancy occurred before age 21 years. Furthermore, parity was negatively associated with the risk of estrogen receptor-positive (ER+) or ER+/progesterone receptor-positive (PR+) while age at first full-term pregnancy was positively associated with the risk of ER+ or ER+/PR+ invasive breast cancer. Neither of these factors was statistically significantly associated with the risk of ER-negative (ER-) or ER-/PR- invasive breast cancer, tests for heterogeneity between subtypes did not reach statistical significance. No clear associations were detected for other pregnancy-related factors.
These results provide some epidemiologic evidence that parity and age at first full-term pregnancy are involved in the development of breast cancer among postmenopausal women. The role of these factors in risk of in situ versus invasive, and hormone receptor-positive versus -negative breast cancer merits further exploration.


This article came out in December of last year. I had heard of this phenomenon before, but I had not seen a study done so tightly until I found this one. Over 52,000 women in California took part in the study, and it found a few associations:

1) The earlier a woman has her first full-term pregnancy, the lower her lifetime breast cancer risk
2) The more pregnancies, the lower her lifetime breast cancer risk.
3) There is an exception to #2: women who have their first child at age 35 and over had the opposite effect: compared to women who had their first child before age 21, they had a 118% increased chance of developing dicta carcinoma in situ, and a 27% increased chance of developing invasive (and likely deadly) breast carcinoma.

This study is one of large-scale association. In many ways, it has stronger statistics than many of the odd cancer theories circulating on the web. This study was done by reputable scientists at a reputable cancer research institute (City of Hope) funded by reputable sources and published in a peer-reviewed journal. It does not, however, contain experiments to determine a mechanism. Those studies are currently underway.

Compared to women in third-world countries, women in first-world countries have less pregnancies and often delay childbirth until later in life. Women in first world countries also develop breast cancer more frequently (2). It is hard not to postulate that the two are related. Keep in mind that these data are all about the incidence (development) of breast cancer and not the treatment of breast cancer. That is another topic.

So, what if you’re a woman who is at increased breast cancer risk, given the article I presented? You need to do this: talk to your doctor. That’s it. For your next physical or check-up just bring it up in conversation. Go to the link and print out the first page of the article if it helps you start a conversation. Your doctor might recommend cancer screening. But the most important thing is to become aware of your personal cancer risks and begin to mitigate them. Barring not ever getting cancer, early detection is the best way to beat the disease.


2) American Cancer Society

Written by Ryon

October 17th, 2011 at 3:06 pm

Posted in Science Blog

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