The body's cells reproduce themselves throughout your lifetime, as tissues wear out and their cells are replaced in a controlled manner. Breast cancer - like all cancers - occurs when that control is lost, and cells begin to divide at an unusually high rate.

No single trigger or cause has been identified for breast cancer. Certain risk factors exist, though, that increase a woman's chance of developing it:

  • age: it's more common in women over 50
  • family history: if a woman's mother or sister had the disease before menopause, this is occasionally associated with one of two genes linked to breast cancer
  • previous breast cancer
  • family history of cervical, uterine, colorectal, or ovarian cancers
  • age of pregnancy: women who haven't had children, or whose first child was born after age 30, run a slightly higher chance of breast cancer than women who had children before age 25
  • age of menstruation: starting periods at a young age - under 12 years old - or entering menopause later - over age 55 - increases breast cancer risks
  • recent research suggests that women who start smoking regularly within five years of the onset of their menstrual periods are 70% more likely to develop breast cancer before the age of 50 than non-smokers. Smoking is linked to breast cancer, and there is also evidence that second-hand smoke is linked to an increased breast cancer risk among pre-menopausal women.
  • alcohol consumption contributes to the risk of breast cancer, with women who consume three drinks a day having a 40% increased risk of this disease
  • hormone replacement therapy (HRT - estrogen plus progesterone): increases the risk of breast cancer slightly after five years of therapy
  • oral contraceptives: increase risks slightly, if used over many years
  • obesity with excess caloric and fat intake

The increased risks of getting breast cancer associated with the above factors are often statistically quite small. Any concerns should be discussed with your doctor.