How Estrogen and Progesterone Impact Breast Cancer

Written by: Dr. Amy Beckley, PhD, Founder and Inventor of the Proov test the first and only FDA-cleared test to confirm successful ovulation at home.

Written on 10/15/21

how estrogen and progesterone impact breast cancer

Here at Proov, we have a passion for all aspects of women’s health and we’re excited to support National Breast Cancer Awareness Month all October long.

Here at Proov, we have a passion for all aspects of women’s health and we’re excited to support National Breast Cancer Awareness Month all October long. Like reproductive health, breast health concerns  each and every one of us. We believe the more information you have for your health journey, the better.

It wouldn’t be Proov, however, to ignore how our hormones can impact breast cancer — after all, we do love hormones! Keep reading to learn more about how estrogen and progesterone may influence breast cancer.

Why are estrogen and progesterone important?

Estrogen and progesterone serve as the two main female reproductive hormones. A healthy cycle starts with a healthy balance between estrogen and progesterone.

In the first half of your cycle — also called the follicular phase — estrogen rises as your body prepares for ovulation. Estrogen thickens your uterine lining to make it a comfy place for an embryo to implant, given that sperm fertilizes an egg.

Estrogen also plays an important role during puberty, by promoting physical changes such as growth of the breasts, pubic and underarm hair, and the start of your cycles. Additionally, estrogen encourages healthy cholesterol levels and preserves bone health, along with impacting things like your brain, mood, and heart, among others.

Then, during the second half of your cycle (the luteal phase), progesterone levels rise and become dominant. After ovulation, the empty follicle from which the egg was released produces progesterone to support implantation and pregnancy.

Progesterone stabilizes the already thick uterine lining and makes it “sticky” enough so an embryo could successfully implant. Progesterone needs to remain elevated to a healthy level for the entire implantation window (days 7-10 past peak fertility) to allow for the best possible chance at successful implantation and pregnancy.

Like estrogen, progesterone also impacts other parts of the body, such as mood, bone health, and sleep patterns. Progesterone can also help relieve symptoms of menopause.

How do our hormones impact breast health?

Our breasts change throughout our life and are impacted by events such as pregnancy, breastfeeding, hormonal changes, weight changes, and age. In fact, our breasts respond to changes in our hormones each cycle.

You may experience changes in your breast texture during your cycle. Estrogen stimulates the growth of milk ducts during your cycle, in preparation for pregnancy. Larger milk ducts can cause your breasts to feel lumpy.

Additionally, some of us may experience swollen, tender breasts right before our periods. Premenstrual breast swelling is caused by an increase in progesterone at the end of the luteal phase, right before your period begins, which stimulates the formation of milk glands. It typically goes away once your period ends.

how estrogen and progesterone impact breast cancer

Some of us may experience swollen, tender breasts right before our period.

How do our hormones impact breast cancer?

There are many rumors floating around about how our hormones impact our risk of developing breast cancer. While some of these rumors are true, others aren’t, so we did our research to get to the bottom of this.

First, both estrogen and progesterone can influence our risk of breast cancer in different ways. In excess, estrogen can lead to a higher risk of breast cancer. Estrogen dominance can cause cells to multiply out of control, which is dangerous and can lead to breast cancer.

Additionally, 7 out of 10 breast cancer cases are hormone receptor-positive, meaning the cancer grows in response to a hormone. Breast cancer can be either estrogen receptor positive or progesterone receptor positive, or both. This means the cancerous cells are sensitive to that hormone and cells with that hormone receptor helps the cancer grow.

While the research surrounding estrogen and breast cancer is pretty clear — estrogen dominance increases the risk of breast cancer — the research surrounding progesterone is less so.

Some research shows that the combination of elevated estrogen and progesterone levels for long periods of time can increase the risk of breast cancer. Other doctors claim that the presence of progesterone can block the dangers of the breast cancer tumor from the immune system, meaning it can take longer for the cancer to be identified and treated.

Other research shows that while synthetic progesterone (also called progestins) has been linked to breast cancer, it’s less clear whether or not bio-identical progesterone can impact your risk. Synthetic progesterone does not match your body’s progesterone in chemical structure, unlike bio-identical progesterone which is the same.

This theory is supported by a different study, which showed that natural (bio-identical) progesterone does not increase the risk of breast cancer. In fact, this same study went even further and showed that progesterone protects against breast cancer.

how estrogen and progesterone impact breast cancer

A study showed that progesterone actually protects against breast cancer.

So, what does this mean for me?

Breast health concerns all people with breasts, regardless of their age or life stage. Since the research shows that hormonal imbalances (specifically estrogen dominance) can increase your risk of breast cancer, better understanding your hormone levels can help you have more informed conversations with your doctor.

While estrogen dominance can be caused by excess estrogen levels during the follicular phase, lower than ideal progesterone levels during the luteal phase can have a similar effect. Estrogen and progesterone must remain balanced throughout your cycle — estrogen levels during the first half of your cycle and progesterone levels during the second half of your cycle need to rise to a similar level.

While our hormones only comprise a small piece of breast health and breast cancer, hormone testing is a small step you can take to better understand your overall status with your doctor. Additionally, we always recommend performing breast self-exams by following the guidance provided by your doctor. If you have concerns about breast cancer, we always recommend consulting your doctor.

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“Hormone balance plays a huge role in fertility, and understanding what’s going on with your hormones doesn’t have to be such a mystery.

It’s actually way easier than most people realize.” 

— Amy Beckley, Proov Founder