Breast density is a measure of tissue composition.
Breasts are made up of fatty tissue, glandular tissue, and connective tissue. Dense breasts have more glandular and connective tissue and less fat.
Dense breast tissue has an abundance of:
- Fibroglandular tissue like glands, ducts, and fibrous connective tissue, with less fatty tissue.
- Stromal (connective) tissue, which overgrows, causing disorganized collagen deposition.
- Excessive collagen buildup and abnormal remodeling make the extracellular matrix stiff and fibrotic.
Higher breast density = higher cancer risk
A study published in Cancer Epidemiology, Biomarkers & Prevention found that women with dense breasts are 4 to 6 times more likely to develop breast cancer compared to women with fatty breasts (PMID: 16775176).
Breast density is driven by hormones…
“We found the one-year change in mammographic density after estrogen plus progestin initiation predicted subsequent increase in breast cancer risk. All of the increased risk from estrogen plus progestin use was mediated through mammographic density change. Doctors should evaluate changes in mammographic density with women who initiate estrogen plus progestin therapy and discuss the breast cancer risk implications” (Byrne et al., 2017)
**It’s important to note that this study looked at a synthetic progestin, medroxyprogesterone acetate (MPA), not bioidentical progesterone. Although MPA binds to progesterone receptors on cells, it does not fully replicate the effects of natural progesterone. In fact, it has been shown to have androgenic and glucocorticoid effects, meaning it behaves more like testosterone and cortisol than progesterone.**
…particularly estrogen.
Estrogen increases breast density by promoting tissue growth and cell proliferation. When estrogen levels are chronically elevated, it can lead to excessive growth of glandular tissue in the breast, which is already highly sensitive to hormonal stimulation.
Estrogen also stiffens breast tissue. It stimulates fibroblasts, the cells responsible for producing collagen, leading to increased collagen deposition. Chronic estrogen exposure can result in abnormal collagen remodeling and a thicker, stiffer, and more fibrotic extracellular matrix. This stiffening contributes to higher breast density and may create an environment more prone to inflammation and cellular changes, both of which are linked to an increased risk of breast cancer.
Other hormonal factors:
- High prolactin: Like estrogen, prolactin stimulates cell growth and proliferation. Prolactin is normally involved in milk and milk gland production, so it naturally increases breast density to enable breastfeeding. However, prolactin levels can also rise in response to stress, low dopamine levels medications, low thyroid hormones, or pituitary disorders. Chronic elevation of prolactin contributes to increased breast density and raises the risk of abnormal tissue changes.
- Low progesterone: Progesterone balances estrogen’s effects by promoting differentiation (maturation) of cells rather than proliferation (growth). When progesterone levels are low, estrogen’s proliferative effects on breast tissue may go unchecked, contributing to increased density and a higher risk of abnormal cell changes.
Breast density increases during pregnancy & breast feeding.
The glands responsible for milk production, a component of dense breast tissue, enlarge during pregnancy and breastfeeding. These are normal changes that help a mother feed her baby.
Prolactin is responsible for these changes, but this is a normal physiological process and not related to cancer risk. When breast feeding stops, the glands return to their normal size during a process called involution.
Dense tissue appears white on a mammogram, which can unfortunately obscure potential abnormalities. Additional screening methods, such as ultrasounds, are often required to obtain a clearer picture of the breast.
Density (usually) decreases with age.
In most women, breast density decreases as they age, because hormone production also usually declines with age. This normally causes glandular and connective tissue to be replaced by fat. However, this isn’t true for everyone.
Some women may maintain relatively high levels of estrogen as they age. Fat cells produce estrogen and some women are exposed to estrogen-mimicking substances that can stimulate breast to tissue. High breast density later in life because estrogen is high.
It is important to maintain healthy progesterone levels, which naturally counteract estrogen’s stimulatory effects. It promotes cell differentiation instead of unchecked growth, allowing breast tissue to regain its normal function and structure. Progesterone also regulates fibroblasts, which produce collagen, reducing the fibrotic stiffness that often accompanies dense breasts.
It’s important to remember that breasts are not static—they are always changing in response to hormonal fluctuations, life stages, and lifestyle factors. During certain times, like pregnancy, breastfeeding, or periods of hormonal shifts, breast density naturally increases. These changes are part of the body’s normal adaptation and don’t necessarily indicate a problem. What matters is the overall pattern of balance in the body.
To manage dense breasts, we have to ensure progesterone levels are sufficient to complement estrogen, and reduce exposure to factors that disrupt this balance, such as stress, poor diet, or environmental toxins.
www.raena.com is a great resource for information on progesterone and hormone balance.