How does age at first full-term pregnancy affect a women's risk for breast cancer?

Updated: Dec 26, 2019
  • Author: Graham A Colditz, MD, DrPH; Chief Editor: Chandandeep Nagi, MD  more...
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Nulliparous women are at more of a risk of breast cancer than parous women. This risk is evident after age 40-45 years, in part owing to the adverse effect of first pregnancy raising incidence among parous women for a number of years when compared to nulliparous women of the same age. [19] In fact, the risk of breast cancer is increased for the first decade following the first pregnancy. The proliferation of breast cells during the first pregnancy results in differentiation into mature breast cells prepared for lactation, but this proliferation may also lead to growth of mutated cells and excess risk over the next decade. Evidence for a similar effect of second and subsequent pregnancies is less clear, perhaps because of the maturation of the breast during the first pregnancy. [19]

The first pregnancy is associated with permanent changes in the glandular epithelium and changes in the biologic properties of the mammary cells. After the differentiation of pregnancy, epithelial cells have a longer cell cycle and spend more time in G1, the phase that allows for DNA repair. The longer the interval from menarche to first pregnancy, the greater the adverse effect of the first pregnancy. [20] The later the age at first full-term pregnancy, the more likely that DNA mistakes have occurred that will be propagated with the proliferation of mammary cells during pregnancy.

The susceptibility of mammary tissue to carcinogens decreases after the first pregnancy, reflecting the differentiation of the mammary gland. This is also seen in the age-dependent susceptibility of the breast to radiation, seen most clearly with the follow-up of women exposed to atomic bomb. [21]

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