Breast cancer is a cancer that forms in the cells of the breasts. Breast cancer is the second most common cancer diagnosed in women in the United States after skin cancer. Breast cancer can occur in both men and women, but it is more common in women.

below are some leading causes of breast cancer..

Symptoms
Signs and symptoms of breast cancer may include:

Nipple discharge
Nipple changes
A breast lump or thickening that feels different from the surrounding tissue
Change in the size, shape or appearance of a breast
Changes to the skin over the breast, such as dimpling
A newly inverted nipple
Peeling, scaling, crusting or flaking of the pigmented area of skin surrounding the nipple (areola) or breast skin
Redness or pitting of the skin over your breast, like the skin of an orange

Causes
Breast cancer occurs when some breast cells begin to grow abnormally. These cells divide more rapidly than healthy cells do and continue to accumulate, forming a lump or mass. Cells may spread (metastasize) through your breast to your lymph nodes or to other parts of your body.

Risk factors
A breast cancer risk factor is anything that makes it more likely you’ll get breast cancer. But having one or even several breast cancer risk factors doesn’t necessarily mean you’ll develop breast cancer. Many women who develop breast cancer have no known risk factors other than simply being women.

Factors that are associated with an increased risk of breast cancer include:

Leading causes of breast cancer:

Being female. Women are much more likely than men are to develop breast cancer.
Increasing age. Your risk of breast cancer increases as you age.
A personal history of breast conditions. If you’ve had a breast biopsy that found lobular carcinoma in situ (LCIS) or atypical hyperplasia of the breast, you have an increased risk of breast cancer.
A personal history of breast cancer. If you’ve had breast cancer in one breast, you have an increased risk of developing cancer in the other breast.
A family history of breast cancer. If your mother, sister or daughter was diagnosed with breast cancer, particularly at a young age, your risk of breast cancer is increased. Still, the majority of people diagnosed with breast cancer have no family history of the disease.
Inherited genes that increase cancer risk. Certain gene mutations that increase the risk of breast cancer can be passed from parents to children. The most well-known gene mutations are referred to as BRCA1 and BRCA2. These genes can greatly increase your risk of breast cancer and other cancers, but they don’t make cancer inevitable.
Radiation exposure. If you received radiation treatments to your chest as a child or young adult, your risk of breast cancer is increased.
Obesity. Being obese increases your risk of breast cancer.
Beginning your period at a younger age. Beginning your period before age 12 increases your risk of breast cancer.
Beginning menopause at an older age. If you began menopause at an older age, you’re more likely to develop breast cancer.
Having your first child at an older age. Women who give birth to their first child after age 30 may have an increased risk of breast cancer.
Having never been pregnant. Women who have never been pregnant have a greater risk of breast cancer than do women who have had one or more pregnancies.
Postmenopausal hormone therapy. Women who take hormone therapy medications that combine estrogen and progesterone to treat the signs and symptoms of menopause have an increased risk of breast cancer. The risk of breast cancer decreases when women stop taking these medications.
Drinking alcohol. Drinking alcohol increases the risk of breast cancer

source: https://myhealthetic.com/2020/02/15/leading-causes-of-breast-cancer/