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• Chemicals Linked to Breast Cancer
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• Timing of Exposure
  • Prenatal
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  • Pregnancy
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  • BPA in Food Packaging
  • State of the Evidence 2010
  • Pollution in People
  • Breast Cancer Endpoints
  • The Falling Age of Puberty
• Chemicals Glossary

Pregnancy and Lactation

Human breasts do not fully mature until a woman's first full-term pregnancy. As the breasts prepare for lactation, the breast structures complete their growth and breast tissue becomes less sensitive to hormones and other chemicals.^[1]

For all racial groups, the younger a woman is when she has her first child, and the more children she has, the less risk she has for developing breast cancer after menopause.^[2] However, for black women, while younger age at first birth and larger family size protects against breast cancer after age 45, it confers additional risk for breast cancer before age 45.^[3],[4] Overall, black women under 45 have higher rates of breast cancer than other racial groups,^[5] and the research relating pregnancy to pre-menopausal breast cancer has not explored social, environmental or economic factors that may influence this racial difference in pre-menopausal breast cancer risk. It is difficult, therefore, to tease apart which aspects of pregnancy—whether it is pregnancy-related risks for breast cancer, higher exposure to environmental chemicals during pregnancy or other social factors—determine this risk.

Because pregnancy means a significant stage in breast maturation, this time period is one in which breast cells divide rapidly and key changes occur in the breast to prepare for breastfeeding.^[6] As a result, pregnancy itself is a vulnerable period for the development of breast cancer. The overall rates of pregnancy-associated breast cancer are small, but pregnancy does confer a small increase in risk that can last for several years after delivery, after which longer term protective effects emerge.^[7] While pregnancy-related breast cancers are relatively rare, tumors that develop at this time can be more aggressive than later-life breast cancers.^[8] Exposure to toxic chemicals during pregnancy may contribute to increased risks of pregnancy-related breast cancer.^[9]

0. Russo J, Russo I (2008). Ch 4: Breast development, hormones. In: Innovative Endocrinology of Cancer, Berstein LM, Santeb RJ (eds.), Landes Bioscience: Austin, TX.
0. Kauppila A, Kyyronen P, Hinkula M, Pukkala E (2009).  Birth intervals and breast cancer risk.  British Journal of Cancer 101:1213-1217.
0. Palmer JB (2003), Wise LA, Horton NJ, Adams-Campbell LL, Rosenberg L. Dual effect of parity on breast cancer risk in African American women. Journal of the National Cancer Institute 95:478—483.
0. Steube AM, Willett WC, Xue F, Michels KB (2009).  Lactation and incidence of premenopausal breast cancer a longitudinal study.  Arch Intern Med. 169:1364-1371.
0. Bowen RL, Stebbing J, Jones LJ (2006).  A review of the ethnic differences in breast cancer.  Pharmacogenomics 7: 935-942.
0. Kauppila A, Kyyronen P, Hinkula M, Pukkala E (2009).  Birth intervals and breast cancer risk.  British Journal of Cancer 101:1213-1217.
0. Lyons TR, Schedin PJ, Borges VF (2009).  Pregnancy and breast cancer:  when they collide.  J Mammary Gland Biol Neoplasia 14:87-98.
0. Mathelin C, Annane K, Treisser A et al. (2008).  Pregnancy and post-partum breast cancer:  a prospective study.  Anticancer Research 28:2447-2452.
0. Andersson TML, Johansson ALV, Hsieh CC, Cnattingius S, Lambe M (2009).  Increasing incidence of pregnancy-associated breast cancer in Sweden.  Obstetrics and Gynecology 114:568-572.