Being pregnant at a young age has long been known to protect women against breast cancer - and a recent study carried out on mice might tell us why.
Study researchers found that the activity of a gene known as Wnt was reduced after pregnancy. Further, relative expression levels of Wnt and another protein Notch were seen to be changed in breast tissue of mice that had given birth compared to virgin mice of the same age.
This is significant because Wnt and Notch work together to control cellular fate. The activity of Wnt is relatively higher in cancer cells - so lowered Wnt after pregnancy may offer a clue as to how pregnancy prevents the runaway cell growth normally seen in breast cancer.
Previous studies have shown that a woman’s risk of developing breast cancer is related to exposure to hormones such as estrogen and progesterone made by her ovaries. Reproductive factors such as early onset of menstruation, late onset of menopause, later age at first pregnancy and never having given birth all increase duration and/or levels of exposure to ovarian hormones - which stimulate cell growth - and are associated with an increase in breast cancer risk.
On the other hand, pregnancy and breastfeeding reduce a woman’s lifetime number of menstrual cycles and exposure to her ovarian hormones. Pregnancy and breastfeeding also cause breast cells to mature so they can produce milk. These differentiated cells may be more resistant to becoming cancer cells.
Some pregnancy-related factors are associated with a reduced risk of breast cancer later in life, such as:
- Early first full-term pregnancy: Women who have their first full-term pregnancy at an early age have a lower risk of developing breast cancer later on in life. For example, in women who have a first full-term pregnancy before the age of 20 years, breast cancer risk is about half that of women whose first full-term pregnancy occurs after the age of 30 years.
- More births: Breast cancer risk declines with number of children. For instance, women who have given birth to five or more children have half the risk of women who have not given birth at all.
- Longer duration of breastfeeding: Breastfeeding for at least a year is associated with a decreased risk of breast cancer.
Other factors that raise breast cancer risk include:
- Older age at birth of first child: The older a woman is when she has her first full-term pregnancy, the higher her risk of breast cancer.
- Recent childbirth: Women who have recently given birth have a short-term increase in risk that declines after about 10 years.