Breast Cancer Risk Factors – Birth Size

Birth size is one of the risk factors for developing breast cancer

Scientists have identified several risk factors that increase a woman’s risk of developing breast cancer by comparing the characteristics of populations of women with and without breast cancer.

Well-established risk factors for breast cancer include increasing age, not having children, and having a late menopause, but another potential risk factor for breast cancer is birth size.

A baby’s weight, length, and head circumference at birth (three related measures of birth size) depend on the levels of hormones (including estrogen, a hormone that often affects breast cancer growth) and other biological factors to which the baby is exposed during pregnancy—its prenatal environment.

The idea that prenatal environment might also affect breast cancer risk in later life was first proposed in 1990, but the findings of studies that have tried to investigate this possibility have been inconsistent.

Researchers re-analyze individual participant data from a large number of studies into women’s health conducted in Europe, Northern America, and China to get more precise information about the association between birth size and breast cancer risk.

The researchers identified 32 published and unpublished studies that had collected information on birth size and on the occurrence of breast cancer. They then obtained the individual participant data from these studies, which involved more than 22,000 women who had developed breast cancer and more than 600,000 women who had not.

Their analyses of the data show that birth weight was positively associated with breast cancer risk in those studies where this measurement was recorded at birth or based on parental recall during the study participant’s childhood (but not in those studies in which birth weight was self-reported or maternally recalled during the participant’s adulthood).

For example, women with recorded birth weights of more than 4 kg or more had a 12% higher chance of developing breast cancer than women who weighed 3–3.5 kg at birth.

Birth length and head circumference were also positively associated with breast cancer risk, but birth length was the strongest single predictor of risk. The amount by which birth size affected breast cancer risk was not affected by allowing for other established risk factors.

Their findings provide strong evidence that birth size, in particular, birth length, is a marker of a woman’s breast cancer risk in adulthood although the mechanisms underlying this association are unclear.

The researchers note that the observed effect of birth size on breast cancer risk is of a similar magnitude to that of other more established risk factors and estimate that 5% of all breast cancers in developed countries could be caused by a high birth size.

Further investigations into how the prenatal environment may affect breast cancer risk might identify new ways to prevent this increasingly common cancer.

Last year, more than one million women discovered that they had breast cancer. In the US, nearly 200,000 women will face the same diagnosis this year and 40,000 will die because of breast cancer.

Put another way, about one in eight US women will have breast cancer during her lifetime. Like all cancers, breast cancer begins when cells acquire genetic changes that allow them to divide uncontrollably and to spread to other organs such as the liver and lungs (metastasize or invasive breast cancer).

This uncontrolled division leads to the formation of a lump that can be detected by mammography (a breast X-ray) or by manual examination of the breasts.

Breast cancer is treated by surgical removal of the lump or, if the cancer has started to spread, by removal of the whole breast (mastectomy). Surgery is usually followed by radiotherapy, chemotherapy, and other treatments designed to kill any remaining cancer cells.

Unlike some cancers, the outlook for women with breast cancer is good. In the US, for example, nearly 90% of affected women are still alive five years after their diagnosis.

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Reference: Silva IdS, Stavola BD, McCormack V, Collaborative Group on Pre-Natal Risk Factors and Subsequent Risk of Breast Cancer (2008) Birth Size and Breast Cancer Risk: Re-analysis of Individual Participant Data from 32 Studies. PLoS Med 5(9): Copyright: © 2008 dos Santos Silva et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License.

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