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Some of the risk factors for breast cancer are listed below.¹ When you read this list, please remember that just because you have one or more of these risk factors, it doesn't mean you will definitely get breast cancer.

• Being older: This is the strongest risk factor. The risk of breast cancer increases with age. It doubles every decade until you reach menopause. To get an idea of your chance of getting cancer in the next 10 years, see How common is breast cancer?
• Starting periods early (before 11) and going through menopause late (after 54): These both increase the risk of breast cancer, perhaps because some breast cancers are encouraged to grow by hormones. If you start your periods early and go through menopause late, then the female hormone estrogen will be around in your body for longer.^{2 3}
• Genes: Most breast cancers happen by chance. But about 5 percent to 10 percent of the women who get breast cancer do so because they've inherited an abnormal gene.⁴ Several genes can increase the risk of breast cancer if they are faulty. The ones that doctors can test for are called BRCA1 and BRCA2.^{5 6} If you have a fault on one of these genes, you have a higher risk of getting breast cancer. But you still might not get breast cancer. Women with a faulty BRCA1 or BRCA2 gene have a 56 percent to 85 percent chance of getting breast cancer.^{7 8} Having an abnormal BRCA2 gene increases the risk of breast cancer in both men and women.
• Family history: Your risk of getting breast cancer may be higher if your mother or sister got it before age 50. Your risk is also higher if you have relatives with both breast cancer and ovarian cancer.
• You've had breast cancer before: If you've had cancer in one breast, you have an increased risk of getting a new cancer in the other breast or in another part of the same breast. However, you'll have regular checks to make sure that any new cancer is picked up early. Some women take tamoxifen to reduce the risk of their cancer coming back. This can also lower your chance of getting a new cancer.⁹ To read more, see Tamoxifen for early breast cancer.
• You've had radiation therapy: Your risk of getting breast cancer is increased if you had radiation therapy to your chest as a child or young adult. You might have had radiation to treat another cancer such as Hodgkin's disease or non-Hodgkin's lymphoma.^{10 11}

You may be able to influence some of these factors.

• Being older than 35 when your first child is born: It's not clear why this should increase a woman's risk of breast cancer. Women who never have children also seem to have an increased risk. In one study, women who had a baby before they were 20 were half as likely to get breast cancer as women who hadn't been pregnant or who had a baby at 35 or older.¹²
• Breast-feeding: Breast-feeding reduces the risk of breast cancer. The longer you breast-feed, the greater the protection.^{13 14}
• Hormone replacement therapy (HRT): Taking HRT increases your risk of breast cancer.² It makes breast tissue denser, and this makes it harder to read a mammogram and diagnose breast cancer from it. To read more, see HRT in our section on menopause.
• Birth control pills: Some researchers have found that while you are taking birth control pills, your risk of getting breast cancer increases by a small amount. But once you stop taking them, this extra risk seems to go away over the next 10 years. Other researchers have found that taking birth control pills don't seem to increase the risk of breast cancer.^{15 16}
• Obesity: After menopause, being obese seems to increase a woman's chance of getting breast cancer if she hasn't taken HRT (see above). This could be because fat helps your body make estrogen, and this seems to encourage some breast cancers to grow. Being obese after menopause can double your risk of breast cancer.¹⁷ A good way to find out whether you are obese is to figure out your body mass index (BMI). This measurement looks at your weight in relation to your height. Many medical organizations say that a BMI of 25 to 29.9 means you're overweight, and a BMI of 30 or more means you're obese. You can use our calculator to figure out your own BMI.
• The food you eat: In groups of people who eat a high-fat diet, women are more likely to die of breast cancer.^{2 18} But there hasn't been enough research to be sure about this.¹⁹ In a recent study, eating a low-fat diet did not reduce the risk of breast cancer among women who had been through menopause.²⁰
• Alcohol: Drinking alcohol slightly increases your risk of breast cancer. One review of 53 studies found that women who have three or four drinks a day increase their risk by about a third. The more you drink, the greater your risk.²¹ The effects of alcohol on your risk of breast cancer may be increased if you're also taking HRT (see above).²² However, your overall risk of breast cancer may still be small even if you drink a lot. Some of the other things we mention on this page have a much bigger impact on your risk.
• Exercise: Lack of exercise may increase your risk of breast cancer.^{23 24 25} But the research on this is mixed.² In one study, walking briskly for as little as one-and-a-quarter hours to two-and-a-half hours each week reduced a woman's risk of breast cancer by 18 percent. Walking 10 hours a week reduced the risk a little more.²
• Smoking: It's not clear if there's a link between smoking and breast cancer. In the past, studies suggested that women who smoked were less likely to get breast cancer than those who didn't. But recent research suggests that it might increase the risk of breast cancer for women who start smoking in early adolescence and smoke for many years.^{26 27}

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.