Effects of Pregnancy on Breast Cancer Risks

(See Biology of the Abortion-Breast Cancer Link)

Early and a repeated number of pregnancies provide protective benefits against breast cancer. During the first half of pregnancy, the proliferation phase, Type 1 and Type 2 lobules increase in number. During the second half of pregnancy (after week 20), cancer-vulnerable Type 1 and Type 2 lobules begin to mature into cancer-resistant Type 4 lobules. After 32 weeks of pregnancy, sufficient Type 4 lobules have developed that a mother is protected against breast cancer, and she incrementally gains the breast cancer risk reduction that will maximize at 40 weeks. After birth and after a mother has lactated and breastfed (or should she choose not to breastfeed), Type 4 lobules regress to Type 3 lobules, which retain the epigenetic changes that protect against cancer’s development.

A woman who has her first full-term pregnancy at age 20 has a 90 percent lower risk of breast cancer than a woman who remains childless or waits until she is 30 for her first full-term pregnancy.1) With each pregnancy after her first, a mother reduces her risk of breast cancer by 10 percent.2) Each year a woman delays pregnancy after age 20, her risk of premenopausal breast cancer increases 5 percent and her risk of postmenopausal breast cancer increases 3 percent.3) This results from the lengthening of the “susceptibility window,” the period between menarche and a first full-term pregnancy, when the breast is composed solely of cancer-vulnerable Type 1 and Type 2 lobules and is most susceptible to carcinogenesis (the formation of normal cells into cancer cells). A woman’s breast cancer risk increases 0.7 percent for each year subsequent births are delayed after the first time she gives birth.4)

Mats Lambe, “Chapter Six: Reproductive Factors,” in Breast Cancer Epidemiology, ed. Christopher I. Li (New York: Springer, 2009), 129-136.
Mats Lambe, Chung-cheng Hsieh, Hsiao-wei Chan, Anders Ekbom, Dimitrios Trichopoulos, and Hans-Olov Adami, “Parity, Age at First and Last Birth, and Risk of Breast Cancer: A Population-Based Study in Sweden,” Breast Cancer Research and Treatment 38, (1996): 305-311.
Françoise Clavel-Chapelon and Mariette Gerber, “Reproductive Factors and Breast Cancer Risk,” Breast Cancer Research and Treatment 72, no. 2 (2002): 107-115.
Adriano Decarli, Carlo La Vecchia, Eva Negri, and Silvia Franceschi, “Age at Any Birth and Breast Cancer in Italy,” International Journal of Cancer 67, no. 2 (July 1996): 187-189.

This entry draws heavily from Induced Abortion and Breast Cancer.