Reproductive History and Cancer Risk
Is there a relationship between pregnancy and breast cancer risk?
Studies have shown that a woman’s risk of developing breast cancer is related to her exposure to hormones that are produced by her ovaries (endogenous estrogen and progesterone). Reproductive factors that increase the duration and/or levels of exposure to ovarian hormones, which stimulate cell growth, have been associated with an increase in breast cancer risk. These factors include early onset of menstruation, late onset of menopause, and factors that may allow breast tissue to be exposed to high levels of hormones for longer periods of time, such as later age at first pregnancy and never having given birth.
Conversely, pregnancy and breastfeeding, which both reduce a woman’s lifetime number of menstrual cycles, and thus her cumulative exposure to endogenous hormones (1), are associated with a decrease in breast cancer risk. In addition, pregnancy and breastfeeding have direct effects on breast cells, causing them to differentiate, or mature, so they can produce milk. Some researchers hypothesize that these differentiated cells are more resistant to becoming transformed into cancer cells than cells that have not undergone differentiation (2, 3).
Are any pregnancy-related factors associated with a lower risk of breast cancer?
Some pregnancy-related factors have been associated with a reduced risk of developing breast cancer later in life. These factors include:
- Early age at first full-term pregnancy. Women who have their first full-term pregnancy at an early age have a decreased risk of developing breast cancer later in life. For example, in women who have a first full-term pregnancy before age 20, the risk of developing breast cancer is about half that of women whose first full-term pregnancy occurs after the age of 30 (4). This risk reduction is limited to hormone receptor–positive breast cancer; age at first full-term pregnancy does not appear to affect the risk of hormone receptor-negative breast cancer (5, 6).
- Increasing number of births. The risk of breast cancer declines with the number of children borne. Women who have given birth to five or more children have half the breast cancer risk of women who have not given birth (7). Some evidence indicates that the reduced risk associated with a higher number of births may be limited to hormone receptor–positive breast cancer.
- History of preeclampsia. Women who have had preeclampsia may have a decreased risk of developing breast cancer (8–11). Preeclampsia is a complication of pregnancy in which a woman develops high blood pressure and excess amounts of protein in her urine. Scientists are studying whether certain hormones and proteins associated with preeclampsia may affect breast cancer risk (8, 12, 13).
- Longer duration of breastfeeding. Breastfeeding for an extended period (at least a year) is associated with decreased risks of both hormone receptor–positive and hormone receptor–negative breast cancers (6, 14).
Is abortion linked to breast cancer risk?
A few retrospective (case-control) studies reported in the mid-1990s suggested that induced abortion (the deliberate ending of a pregnancy) was associated with an increased risk of breast cancer. However, these studies had important design limitations that could have affected the results. A key limitation was their reliance on self-reporting of medical history information by the study participants, which can introduce bias. Prospective studies, which are more rigorous in design and unaffected by such bias, have consistently shown no association between induced abortion and breast cancer risk (20–25). Moreover, in 2009, the Committee on Gynecologic Practice of the American College of Obstetricians and Gynecologists concluded that “more rigorous recent studies demonstrate no causal relationship between induced abortion and a subsequent increase in breast cancer risk” (26). Major findings from these studies include:
- Women who have had an induced abortion have the same risk of breast cancer as other women.
- Women who have had a spontaneous abortion (miscarriage) have the same risk of breast cancer as other women.
- Cancers other than breast cancer also appear to be unrelated to a history of induced or spontaneous abortion.
Syndicated Content Details:
Source URL: https://www.cancer.gov/node/14370/syndication
Source Agency: National Cancer Institute (NCI)
Captured Date: 2013-12-06 15:00:06.0