Breast cancer is one of the most common cancers affecting women, and many types are hormone-sensitive—meaning they grow in response to estrogen and progesterone. Understandably, this has led to concern that taking hormone therapy, whether as birth control or during menopause, could raise the risk of developing breast cancer.

But the evidence tells a more nuanced story.

Large studies, including the Women's Health Initiative, have shown that menopausal hormone therapy (MHT)** does not significantly increase breast cancer risk—and in some cases, it may even reduce it. Women taking estrogen-only MHT after hysterectomy, for example, had a lower risk of both developing and dying from breast cancer. Other research shows that women who were on MHT before being diagnosed had better survival rates.

Adding to the complexity is a fascinating historical paradox: decades ago, doctors used either high-dose oestrogen or ovary suppression to treat advanced breast cancer—with positive results. This seeming contradiction highlights how much our understanding of breast cancer and hormones has evolved.

We now know two key things that help explain these findings:

Most breast cancers are thought to originate in breast stem cells, which can be triggered into malignancy under certain conditions.
 

Breast tissue—especially breast fat and even tumours—can produce their own oestrogen, often in the form of estrone, which may have a stronger influence on tumour growth than the estradiol used in MHT or the ethinyl estradiol in contraception.
 

In other words, it's not as simple as "more oestrogen equals more cancer." The relationship between hormones and breast cancer is far more complex—and the latest research suggests that MHT is safer than many women have been led to believe.

So What Increases Your Risk Of Breast Cancer

High Breast Cancer Risk Factors

These factors increase the risk of breast cancer by 4x the baseline risk:

Moderate Breast Cancer Risk Factors

These factors increase the risk of breast cancer 2x greater than the baseline risk:

Minimal Breast Cancer Risk Factors

These factors increase the risk of breast cancer by less than 2x greater than the baseline risk:

How Breast Cancer Begins

In the past 30 years, our understanding of how breast cancer starts and progresses has evolved significantly. Two key discoveries have shifted the way scientists think about this disease:

1. Most breast cancers are thought to originate from breast stem cells, which drives the growth, spread, and resistance to treatment of breast cancers.

2. The breast itself—particularly breast fat and even the tumour—can produce estrogen locally, and this local production alongside other inflammatory mediators play a bigger role in cancer development than hormones circulating in the bloodstream or taken as hormone therapy.

What Are Breast Stem Cells?

Breast stem cells are low in number ( 1 per 200-1,000 breast cells) and usually lie dormant, quietly maintaining breast tissue. These cells are estrogen and progesterone receptor negative (ER−PR−), meaning they don’t respond directly to oestrogen or progesterone. 

Breast stem cells can produce off-spring cells, which do respond to hormones and eventually form the duct and lobular cells where most breast cancers develop.

Importantly, breast stem cells are long-lived, so over time they can accumulate enough damage—either from genetic mutations, environmental exposures, or hormonal signals—to escape the usual control mechanisms. This is when cancer can begin.

Breast Cancer Risk Factors Can Begin In Early Life

Current research suggests that breast cancer risk may be shaped early—starting even before birth. 

For example:

• Birth weight is linked to the number of fetal stem cells (higher birth weight may mean more stem cells).
   

• Early-life nutrition, growth, and body size—which are connected to hormones like insulin and IGFs (insulin-like growth factors)—are all predictors of breast cancer risk later on.
   

These findings point to a “life-course” model of breast cancer, where factors from the womb through early adulthood help shape future risk.

How Breast Tissue Develops—and Why It Matters

Most breast cancers start in small structures called terminal duct lobular units (TDLUs)—not the ducts themselves. These units go through different stages of development, from Lob 1 to Lob 4.

• Lob 1 and Lob 2 are the more immature forms and are the predominate forms in women who haven’t had a full-term pregnancy.
   

• Lob 3 and 4 are more developed and usually appear only after a first full-term pregnancy.
   

How developed the breast tissue is can influence breast cancer risk. Lob 1 is more sensitive to oestrogen and has a higher rate of cell growth, making it more likely to develop cancer. Lob 3 and 4 are more stable and less likely to turn cancerous.

Pregnancy’s Role in Breast Cancer Risk

Pregnancy is a time of incredibly high estrogen and progesterone hormones. It has both protective and risk-enhancing effects on breast cancer depending on age of first pregnancy. 

Having a first full-term pregnancy at a younger age (around 25) can halve breast cancer risk. This is thought to be due to pregnancy triggering the complete development of mature, less cancer-prone lobules (Lob 3 and 4) and reduces the number of vulnerable breast stem cells. 

For women over 35, pregnancy can slightly increase the risk—possibly by stimulating pre-existing abnormal cells. 

Breastfeeding also offers a slight additional reduction in risk.

Women with breast cancer who have a subsequent pregnancy, have a lower risk of breast cancer recurrance and longer cancer-free survival (Pregnancy After Breast Cancer 2021). 

Early Breast Cancers and the Role of Hormone Therapy

Autopsy studies have revealed that a surprising number of women have undetected breast cancers, particularly ductal carcinoma in situ (DCIS) —early-stage lesions that often don’t show up on mammograms (8.9% of women). These were often far too small to be detected by mammogram. 

In women aged 40–70, the prevalence was notably higher, suggesting a substantial reservoir of early breast cancers in the population.

Some researchers propose that menopausal hormone therapy (MHT) doesn’t cause breast cancer but may stimulate growth in small, existing tumours—especially those taking MHT with synthetic progestins. 

Estrogen Is Made Inside the Breast And Drives Tumour Growth

Although hormone-sensitive breast cancer cells (ER+PR+) respond to estrogen, MHT doesn't appear to significantly fuel their growth. 

This is because the main source of estrogen influencing these tumours is made locally within the breast—not from hormones circulating in the bloodstream.

Breast fat and tumour tissue produce high levels of estrogen, especially through enzymes that they express like aromatase and sulphatase. Estradiol levels in breast tissue can be up to 20 times higher than in the blood or what can be achieved with MHT.

Around 70% of ER+ breast cancers have increased aromatase activity, making their local hormone environment particularly active.

Other local factors—like additional growth signals, inflammation, and new blood vessel formation—also support tumour growth, making the tumour microenvironment a key player in cancer progression.

Progestins, Progesterone, and Breast Cancer Risk

Not all progestogens act the same in breast tissue.

Synthetic progestins used in some MHT regimens are linked to a slightly higher breast cancer risk compared to oestrogen-only or natural progesterone formulations. 

The French E3N study found that MHT containing natural progesterone (body-identical progesterone such as utrogestan) was associated with lower risk than regimens using synthetic progestins (E3N cohort study, 2007)

Interestingly, high-dose medroxyprogesterone acetate (MPA)—a synthetic progestin—can actually treat some breast cancers by suppressing hormone receptors.

The research suggests natural progesterone at normal levels doesn’t cause breast cancer, but may promote growth in existing hormone-sensitive tumours. 

Lab studies show that adding progestins can expand cancer stem cells, which slightly increases tumour growth.

Menopause Hormone Therapy (MHT) and Breast Cancer Survival

In the estrogen-only arm of the WHI, women had a lower risk of developing and dying from breast cancer. The combined MHT (oestrogen + progestin) showed a slight increase in the discovery of breast cancer, but no increase in death rates, possibly due to the way progestins affect cancer stem cells.

A separate review by Yu et al. found that MHT use was linked to better survival after a breast cancer diagnosis—especially for current users. Their tumours were more likely to be lower grade and hormone receptor-positive, which may help explain the improved outcomes.

Does MHT Cause Breast Cancer? Understanding the Real Risk

Breast cancer likely begins when long-lived breast stem cells accumulate genetic damage over decades, for those with ER+ tumours, these cancers are able to grow in response to oestrogen. The main source of this oestrogen is produced locally within breast fat and breast cells, not from hormone therapy.

Low-dose synthetic progestins may slightly stimulate cancer stem cells, possibly promoting the growth of existing, undetected tumours. This may explain the slight increase in breast cancer risk seen in the combined oestrogen–progestin arm of the WHI study—especially in older women. In contrast, oestrogen-only therapy was linked to a reduced risk, possibly due to its effects on tumour cell death.

The Key Messages About Hormones And Breast Cancer:

MHT does not cause breast cancer, but in some cases, it may bring forward the diagnosis of an existing tumour.

Estrogen-only MHT appears to be safer for the breast than combined therapy, and natural progesterone is likely safer than synthetic progestins. Reassuringly, women on MHT at the time of breast cancer diagnosis tend to have better survival outcomes.

Always talk to your doctor about what is the best treatment options for you when it comes to both contraceptive and menopause hormone therapy advice. 

**Menopause Hormone Therapy should be prescribed by trained medical professionals. It is licenced to treat the symptoms of natural or surgically induced menopause. Estradiol is also licenced for the prevention of postmenopausal osteoporosis. Ask your doctor about the benefits and risks of using MHT and whether it is right for you. If you have any side effects or concerns, speak to your health professional. MHT has benefits and risks. Always follow your doctor’s instructions. For more information refer to the Consumer Medicine Information for estradiol/estriol/progesterone at www.medsafe.govt.nz.