Breast cancer develops when genes undergo changes that make a cell grow out of control. Some of these genetic mutations are inherited (passed down from parent to child), while others develop over the course of a person’s life, after cells become damaged.

Several breast cancer risk factors can create damage within a cell and increase a person’s chances of developing cancer in the breast tissue. Some of these risk factors are related to menstruation (getting your period) and reproductive history (whether you have gotten pregnant). Experts believe that hormones play a role in the link between these risk factors and breast cancer.

The ovaries make the female sex hormones estrogen and progesterone. These hormones are important in helping control several different processes in the body, including menstruation and pregnancy. However, estrogen and progesterone may cause damage to cells over time. The longer breast tissue is exposed to these hormones, the more likely cells are to turn cancerous.

Factors That Affect Breast Cancer Risks

The ages at which you start and end menstruating, and whether you become pregnant and breastfeed, affect how many menstrual cycles you’ll have. Factors that lead to fewer menstrual cycles lead to less hormone exposure and can lower breast cancer risk.

Additionally, medications that contain hormones, including oral contraceptives (birth control pills) and hormone replacement therapy (treatments to relieve menopause symptoms) may raise a person’s chances of developing breast cancer.

Following are some menstruation and pregnancy factors that may affect one’s chances of developing breast cancer.

Age at First Menstruation

Women who began menstruating before the age of 12 are more likely to develop breast cancer, according to Breastcancer.org.

Age at Menopause

Menopause occurs when the ovaries stop producing hormones. At this point, a person no longer has a period and cannot become pregnant. In the U.S., the average age of menopause is 51.

Because having more menstrual cycles increases the chances of developing breast cancer, people who go through menopause later in life have an increased breast cancer risk. Women are thought to have a higher risk if they stop having periods after turning 55, according to Breastcancer.org.

Giving Birth

Giving birth slightly decreases a woman’s chance of developing breast cancer over the course of their life, according to the American Cancer Society. Additionally, having multiple pregnancies further reduces risk.

However, giving birth can temporarily increase one’s risk of breast cancer. Cancer research studies have found that a woman’s likelihood of developing this condition slowly increases after giving birth, reaching a peak after about five years. At this time, a woman’s risk of breast cancer is 80 percent higher compared to the risk of another person who hasn’t given birth, according to an analysis from the Annals of International Medicine. After five years, the risk level steadily decreases over time. Thirty-four years after giving birth, the risk for a woman who’s given birth is 23 percent lower, compared to someone who hasn’t had a child.

Although pregnancy raises breast cancer risk for several years, the chances of developing the condition in the reproductive years are low to start with. Half of women with breast cancer in the U.S. are 63 years old or older by the time they are diagnosed, according to the U.S. National Cancer Institute Surveillance, Epidemiology, and End Results Program.

Most of this increased risk after birth comes from estrogen receptor (ER)-negative breast cancer. Estrogen receptor is a protein found on the surface of some breast cancer cells. ER helps fuel cancer cell growth. When breast cancer cells don’t have ER, they are called ER-negative.

Women who have given birth have a slightly increased risk of developing ER-negative breast cancer throughout their lives, according to the Annals of Internal Medicine analysis. However, ER-negative breast cancers are much less common than ER-positive ones, and so the overall risk of developing any type of breast cancer is still lower for a woman who has given birth.

There are a few possible reasons why pregnancy can help prevent breast cancer:

• Menstruation doesn’t occur during pregnancy.
• Pregnancy allows the breast tissue to get rid of cells that may be damaged.
• Breast cells don’t fully mature until pregnancy, and mature breast cells may be less likely to grow abnormally and turn into tumors.

Age at First Pregnancy

Being pregnant and giving birth leads to changes in the breast tissue that help lower cancer risk. When these changes occur earlier, they can start protecting against breast cancer sooner.

The age at which a woman has her first full-term pregnancy affects breast cancer risk, according to the Annals of Internal Medicine analysis:

• Women who give birth before the age of 25 have a slightly reduced risk of breast cancer, compared with women who never had children.
• Women who have their first birth between the ages of 25 and 34 are 25 percent more likely to develop breast cancer four and a half years later. After about 13 years, they become less likely to develop breast cancer, compared to women who have never given birth.
• Women who give birth over the age of 35 have a 40 percent higher risk of breast cancer six and a half years later. Over time, their risk slowly normalizes, becoming similar to the level of risk for women who have never had children.

Breastfeeding

Women who breastfeed are slightly less likely to be diagnosed with breast cancer than those who don’t. Those who breastfeed for at least one year have the most protection.

Experts think that one reason there is a link between breastfeeding and breast cancer risk is that breastfeeding delays periods from returning. This means that someone has fewer menstrual cycles and is exposed to fewer hormones. Additionally, when breast cells are continually making milk, they are less likely to undergo cancerous changes.

People also typically eat a healthier diet, stop drinking, and avoid smoking while breastfeeding. These lifestyle changes can help lower breast cancer risk.

Breast Cancer Prevention

You can’t control factors such as how old you are when you get your first period and when you start menopause. You may be able to choose to have children earlier in life or to breastfeed, but this isn’t always an option for everyone. Some may not be able to have children, or may choose not to. However, there are other ways you can help protect yourself against breast cancer.

In order to keep your risk of breast cancer as low as possible, you can:

• Get more physical activity throughout your day.
• Eat more fruits, vegetables, and dairy, and fewer red and processed meats.
• Maintain a healthy weight.
• Drink less alcohol, or avoid it completely.
• Get a mammogram (breast cancer screening test) every two years starting at age 50, or sooner if your doctor recommends it.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 56,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with breast cancer? Share your experiences in the comments below, or start a conversation by posting on your Activities page.