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

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Are any pregnancy-related factors associated with an increase in breast cancer risk?

Some factors related to pregnancy may increase the risk of breast cancer. These factors include:

Older age at birth of first child. The older a woman is when she has her first full-term pregnancy, the higher her risk of breast cancer. Women who are older than 30 when they give birth to their first child have a higher risk of breast cancer than women who have never given birth.15

Recent childbirth. Women who have recently given birth have a short-term increase in breast cancer risk that declines after about 10 years. The reason for this temporary increase is not known, but some researchers believe that it may be due to the effect of high levels of hormones on the development of cancers or to the rapid growth of breast cells during pregnancy.16

Taking diethylstilbestrol (DES) during pregnancy. DES is a synthetic form of estrogen that was used between the early 1940s and 1971 to prevent miscarriages and other pregnancy problems. Women who took DES during pregnancy may have a slightly higher risk of developing breast cancer than women who did not take DES during pregnancy.17 Some studies have shown that daughters of women who took DES during pregnancy may also have a slightly higher risk of developing breast cancer after age 40 than women who were not exposed to DES while in the womb,18 but the evidence is inconsistent.19

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.