Estrogen is a hormone produced mainly in the ovaries. It’s important in sexual development, the menstrual cycle and pregnancy. A woman's lifetime exposure to estrogen is related to her risk of breast cancer. In this section, we discuss some estrogen-related risk factors that can have a direct effect on risk, including hormone use during menopause and birth control pills.

Starting your period before age 12 means a longer lifetime exposure to estrogen, which is linked to a small increased risk of breast cancer. For example, women who begin their periods before age 11 have about a 15 to 20% higher breast cancer risk of breast cancer compared to those who begin their periods at age 15 or older. Reaching menopause at a later age increases a woman's lifetime exposure to estrogen, which is linked to a higher risk of breast cancer. Studies show women who go through menopause after age 55 have about a 40% higher risk of breast cancer than women who go through menopause before age 45. Women with higher levels of estrogen in their blood after menopause have a higher risk of breast cancer than those with lower levels. To learn more, you can visit the Risk Factors section on komen.org.

The relationship between having children and breast cancer risk is complex. A first pregnancy is linked to an increased short-term risk, but it lowers the risk in the long run. The impact of these risks depends on a woman’s age at the time of her first pregnancy. Women who have their first child at age 35 or younger generally have a lower risk of breast cancer. While the risk of breast cancer is higher for about 10 years after giving birth, these women have a lower risk of breast cancer than those who never had children. Women who have their first child after age 35 have a slightly higher risk of breast cancer than women who don’t have children or those who have children at younger ages. For these cases, the short-term increase in risk from a first pregnancy is never fully offset by the long-term protective effects of childbirth. In general, the more children a woman has, the lower her risk of breast cancer tends to be. After the first child, each additional birth is linked to a small, additional reduction in risk. Breastfeeding after pregnancy is also linked to a lower risk of breast cancer, particularly before menopause. Many studies indicate that women who breastfed for a lifetime total of 1 year were less likely to develop breast cancer than those who never breastfed. The longer a woman breastfeeds, the greater the benefits may be. To learn more, you can visit the Risk Factors section on komen.org.

Current or recent use of birth control pills is linked to a slight increase in breast cancer risk. However, this extra risk has a fairly small impact because most young women have a low overall risk of breast cancer. The extra risk declines when women stop taking the pill and returns to normal about 10 years later. Other types of birth control that contain hormones, including Depo Provera, IUDs that release hormones, the birth control patch and the vaginal ring are under study to determine whether they are linked to breast cancer risk. Lastly, women who take menopausal hormone pills with both estrogen and progestin have a higher risk of breast cancer. This risk increases the longer women take menopausal hormone therapy. However, menopausal hormone therapy, or MHT, is approved by the U.S. Food and Drug Administration (also known as the FDA) for the short-term relief of hot flashes and other menopausal symptoms. The FDA advises that women who take menopausal hormone pills take the lowest dose possible for the shortest amount of time needed. Women who start menopausal hormones before age 60 appear to have more benefits and fewer risks with MHT.

Also, when women stop taking estrogen plus progestin, their risk of breast cancer starts to decline. Within about 5 to 10 years, their risk returns to a level of a woman who has never used menopausal hormone therapy. Women who take menopausal hormone pills containing only estrogen for 10 or more years also have an increased risk of breast cancer. However, vaginal hormone therapies do not appear to increase the risk of breast cancer. When considering taking menopausal hormone therapy, women should discuss the risks and benefits of these drugs with their health care providers. To learn more, you can visit the Risk Factors section on komen.org.