What you need to know about hormones and breast cancer

Key Takeaways:

• Increased exposure to both natural and synthetic sources of estrogen and progesterone is strongly associated with increased risk of breast, uterine, and ovarian cancers. The link is strongest with breast cancer. 
• Lifestyle factors, such as timing of births, birth control, and hormone treatments for perimenopause/menopause can influence cancer risk.
• Catch incorporates the latest and most compelling scientific research to pinpoint–and continually modify–your unique risk profile. 

Breast cancer is on the rise in recent years, specifically in younger women. In this article we explore the emerging understanding of how hormones–and major shifts in those hormones, such as menopause–impact cancer risk.

The relationship between hormones and cancer is complicated, but generally speaking, higher exposure to estrogen and progesterone correlates with higher risk of reproductive cancers. Breast cancer is the most clearly impacted, but increased exposure also corresponds with increased risk of uterine and ovarian cancers. This includes exposure to endogenous hormones–the ones created in one’s own body.

The role these hormones play in women’s health is still being studied, but research to date points to several clear conclusions. Here’s the current state of the science on how hormones might impact your personal risk:

Getting your period early raises your risk for cancer

Exposure to estrogen and progesterone increases one’s risk of breast cancer, no matter the source of those hormones, and the onset of one’s period marks a massive uptick in the amounts that are produced by the body.

An early period (before age 12) means higher lifetime exposure, which means a higher risk of estrogen-impacted cancers. Studies have shown that girls have started menstruating earlier, and rates of early and very early menstruation have more than doubled in the last several decades. Possible explanations include increased rates of childhood obesity, exposure to endocrine-disrupting chemicals, and exposure to pollutants.

The impact of oral contraceptives and IUDs is mixed

Oral contraceptives rely on synthetic versions of estrogen and/or progesterone to inhibit ovulation and conception. This may be why oral contraceptives have been associated with increased rates of breast and cervical cancers, particularly among current and longtime users (though the correlation isn’t consistent, and risk appears to decline once users quit the medication).

There is much stronger evidence that these medications decrease one’s risk of ovarian and uterine cancers, possibly by suppressing cell proliferation and reducing lifetime ovulation events.
IUDs may impact breast cancer, but studies have shown conflicting results. The impact of IUDs on ovarian and cervical cancers is likewise still unclear. There’s good evidence, however, that they reduce the risk of uterine cancers.

The mixed effect birth control has on cancer risk–increasing the risk of some cancers while decreasing the risk of others–can make it confusing to know the “best” health decision to make. Knowing your unique risk for these cancers can help you to weigh the pros and cons of these methods for you, personally. Catch assesses all the latest emerging science to determine which results are reliable and relevant, and uses it to determine–and continually modify–your personalized risk of cancer.

Pregnancy and breastfeeding reduce risk (but when you have a child matters)

All women experience a period of heightened risk for breast cancer for around 10 years after giving birth. Cellular changes in the breasts and exposure to estrogen and progesterone are the likeliest explanation for this temporary risk increase.

For long term risk, when you have your first child matters. An early first pregnancy–before the age of 22–appears to significantly reduce lifetime breast cancer risk. One possible explanation is that the differentiation in breast cells during and after pregnancy makes them more resistant to carcinogenesis in the long term.

Late first pregnancy, on the other hand–after the age of 35–has been shown to increase risk relative to women who never give birth. It’s possible this is also due to the changes in the breasts during and after pregnancy. As we age, DNA changes that can develop into cancer become more likely. The rapid breast cell proliferation during and after pregnancy leads to damaged DNA spreading more quickly. For younger mothers, the temporary increase in risk is outweighed by the long-term benefit of more cancer-resistant breast cells, but for older mothers, the balance tilts towards higher risk overall.

Breastfeeding–which can suppress estrogen–is associated with risk reduction no matter when a woman gives birth. Longer duration of breastfeeding correlates with greater risk reduction.

Women who have multiple children, particularly if those children are spaced close together, appear to reduce their risk even more. Reduced exposure to endogenous estrogen after childbirth (a pro-growth and pro-cancer hormone) is the likeliest explanation.

Endometriosis and polycystic ovary syndrome (PCOS)

While evidence is not definitive, several studies and meta-analyses have found that endometriosis (an often painful condition in which endometrial tissue grows outside the uterus) and polycystic ovary syndrome (PCOS) may both increase breast cancer risk. The links between these conditions are still being studied, and in recent years, both these conditions are getting more research focus.

Possible explanations for the observed risk increase include increased estrogen levels with both endometriosis and PCOS, chronic inflammation caused by the syndromes, and overlapping risk factors (such as an early first period, which also appears to increase the risk of endometriosis).

Infertility may increase your risk of breast cancer

While the link between infertility and breast cancer requires more research, multiple studies have found that infertility–particularly primary infertility–correlates to a higher risk of postmenopausal breast cancer. Possible explanations include the fact that infertility often delays first childbirth (sometimes significantly), the use of hormone therapies to help encourage pregnancy or as part of IVF, and underlying syndromes as the cause of infertility–such as endometriosis and PCOS–that may separately impact breast cancer risk.

Late menopause means higher risk (and be thoughtful of how you handle perimenopause symptoms)

Late menopause–after the age of 55–has been tied to higher rates of breast cancer, possibly due to increased lifetime exposure to estrogen and progesterone.

Separately, hormone replacement therapy (HRT) used to treat symptoms of perimenopause and menopause has been shown to increase risk, but to a relatively small degree. Combination therapies (with both estrogen and progesterone) raise risk more than estrogen-only treatments, and length of use matters–longer use means more risk increase.

If you’re considering HRT to treat symptoms of either perimenopause or menopause, be sure to weigh the potential treatment options–and how they might impact your overall risk–with your doctor.

Excess body weight

Excess body weight, particularly after menopause, has been conclusively tied to increased breast cancer risk. Excess fat tissue can increase the amount of estrogen the body produces, particularly after menopause (when ovarian production dwindles). This increased exposure is the likeliest explanation for the risk increase, though systemic inflammation and insulin resistance may also contribute to the development and growth of cancer.

As you consider your cancer risk in light of major hormonal shifts like childbirth and menopause, keep in mind that the impact of hormones on cancer risk is complex, and our understanding of the role hormones play in carcinogenesis is always evolving. Catch is constantly evaluating the latest clinical research to separate what’s relevant from what’s simply headline-grabbing. As a part of your Catch membership, you receive a personalized Risk Assessment across 21 cancers (including breast, ovarian, and uterine cancer) that is regularly updated to include the most compelling cutting edge research, and translate it into actionable information for our members. With Catch, you can trust that you’re getting the most up-to-date understanding of your unique risk.

You can’t control everything that influences your risk for reproductive cancers, but understanding the impact of major risk factors is the first step to taking charge of your own health.

Sources:

1. Menstrual History https://www.breastcancer.org/risk/risk-factors/menstrual-history
2. Reproductive History and Cancer Risk https://www.cancer.gov/about-cancer/causes-prevention/risk/hormones/reproductive-history-fact-sheet#r7
3. Oral Contraceptives and Cancer Risk https://www.cancer.gov/about-cancer/causes-prevention/risk/hormones/oral-contraceptives-fact-sheet
4. Some Hormonal IUDs Increase Breast Cancer Risk https://www.breastcancer.org/news/hormonal-iuds-increase-breast-cancer-risk
5. Association between intrauterine device use and endometrial, cervical, and ovarian cancer: an expert review https://www.ajog.org/article/S0002-9378(23)00224-7/
6. Breast Cancer Risk Factors: Age at First Childbirth and Number of Childbirths https://www.komen.org/breast-cancer/risk-factor/age-at-first-childbirth/
7. History of infertility and risk of breast cancer: a prospective cohort study https://pmc.ncbi.nlm.nih.gov/articles/PMC10695171/
8. Risk of endometrial, ovarian and breast cancer in women with polycystic ovary syndrome: a systematic review and meta-analysis https://pubmed.ncbi.nlm.nih.gov/24688118/
9. The association between endometriosis and risk of endometrial cancer and breast cancer: a meta-analysis https://pmc.ncbi.nlm.nih.gov/articles/PMC9673303/
10. Estrogens and breast cancer: mechanisms involved in obesity-related development, growth and progression https://pmc.ncbi.nlm.nih.gov/articles/PMC6502693/
11. Menarche and Time to Cycle Regularity Among Individuals Born Between 1950 and 2005 in the US https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2819141