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.