Knowing the risk factors that contribute to breast cancer can help you evaluate your likelihood of getting the disease. In some cases, you can even make changes now that will improve the chances of avoiding it. Here's what researchers know about breast cancer risks.

Menstruating longer, having children late

Breast cancer risk is affected by changes in hormone levels throughout life. Longer periods of high estrogen levels lead to a higher risk of breast cancer -- for example, women who had their first period before the age of 12, entered menopause later than 55, and either didn't have children or had their first child when they were older than 30. In contrast, if your mother became pregnant at an early age or had multiple children, this decreases her risk of breast cancer. Similarly, breastfeeding for one and a half or two years affects a woman's hormone levels and thus decreases her breast cancer risk.

Close relatives with breast cancer

If your mother has a first-degree relative (mother, sister, or daughter) with breast cancer, it doubles her risk, while having two first-degree relatives gives her five times the risk of someone with no family history of the disease. Statistics can be confusing, though; while 20 to 30 percent of women with breast cancer have a family member with the disease, 70 to 80 percent of those with breast cancer had no family history to alert them.

Genetic predisposition

Studies show that between 5 and 10 percent of breast cancer cases are hereditary. Those women with BRCA1 and BRCA2 gene mutations, which seem to run in families, have an 80 percent chance of getting breast cancer during their lifetime. Jews of Eastern European descent are most likely to carry the BRCA mutations, but they also occur in women from Hispanic, African American, and European origins. A number of less well-known genes, including the ATM, CHEK2, and p53 genes, also increase the risk of breast cancer.

Use of hormone-replacement therapy

The results of long-term studies show that women who took a combination of estrogen and progesterone to treat menopausal symptoms have both an increased risk of getting breast cancer and an increased risk of dying from breast cancer. However, the risk seems to apply only to women who are currently taking hormones or who stopped taking them within the past few years; if your mother stopped taking hormones five or more years ago, her risk level will have returned to that of the general population. Estrogen-only therapy doesn't seem to increase breast cancer risk except in women who took the hormones for more than ten years. In this case, estrogen therapy increases the risk of both breast and ovarian cancer.

Being overweight

Experts are still trying to understand the connection between obesity and breast cancer risk, but it's clear that being significantly overweight, especially in the midsection, increases breast cancer risk in postmenopausal women. Estrogen is stored in fat tissues, so after menopause, when the ovaries are no longer producing estrogen, having excess fat leads to higher estrogen levels. If your mother was thin as a younger woman but gained weight later in life, it's of more concern than if she was heavy all along.

A sedentary lifestyle

Physical activity has a direct effect on breast cancer risk. One Women's Health Initiative study showed that walking as little as one to two and a half hours per week lowered a woman's breast cancer risk by 18 percent. If your mother is physically inactive, she's at higher risk of developing breast cancer.

Secondhand smoke

While smoking is a risk factor for many types of cancer, it doesn't appear to raise the risk of breast cancer significantly. Oddly, though, breathing secondhand smoke has been linked with an elevated risk of breast cancer. Studies have shown the greatest risk from secondhand smoke is for younger, premenopausal women. This is an area researchers are still trying to understand.

Drinking alcohol

While the effect of heavy alcohol use on the liver is fairly well known, it's less well known that regular drinking increases breast cancer risk as well. If your mother has two to five drinks a day, she has one and a half times the risk of women who don't drink. Having one alcoholic drink a day raises her risk slightly but not a lot.

"Benign" breast conditions

If your mother has ever had an abnormal breast biopsy, this may indicate an increased breast cancer risk. Cysts and fibrosis have not been linked with elevated breast cancer risk, but conditions that come under the heading of "proliferative lesions" raise breast cancer risk one to two times higher than normal, while a condition known as "hyperplasia" raises breast cancer risk four to five times higher than normal.

Low sun exposure and lack of vitamin D

While sun exposure has long been considered a risk factor for skin cancer, the opposite appears to be true for breast cancer. Research published in the fall of 2007 revealed a surprise: Sun exposure appears to decrease the risk of advanced breast cancer. Women with high sun exposure (defined for the study as having darkened skin on the forehead) had half the risk of developing advanced breast cancer -- cancer that has spread beyond the breast -- as women with low sun exposure. Researchers explain this phenomenon by the fact that sun exposure increases vitamin D production, and there's increasing evidence that vitamin D is important in slowing the growth of breast cancer cells.

Low melatonin and shift work

Studies show that women who worked night shifts face a significantly higher risk of breast cancer -- up to 60 percent higher, according to one study published by the National Cancer Institute. The thinking is that nighttime work, which entails exposure to light at night, interferes with the body's production of melatonin, the sleep-inducing hormone. Sleep deprivation and melatonin interruption in turn stimulate increased estrogen production, researchers believe. Additional studies have linked low melatonin levels in general with breast cancer. The connection is strong enough that researchers are exploring whether taking supplemental melatonin can protect against breast cancer.

A diet high in fat (especially for breast cancer survivors)

The jury is still out on fat and breast cancer; one study published in 2003 found that women who ate a diet high in fat -- particularly saturated fat from meat -- had double the breast cancer risk of women who ate a low-fat diet. However, other studies haven't found a definitive link. What's clearer is that women who've had one bout of breast cancer can lower their risk of recurrence by switching to a low-fat diet. Organizations such as the National Cancer Institute are still studying the connection between diet and breast cancer, but they recommend a low-fat diet for all those concerned about their risk of cancer.

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