We had a guest over for dinner who's step-mother had breast cancer some years ago. It was also mentioned that this woman had never had children, and both our guest and my wife related that they had the understanding that women who have never had children have an increased likelihood of getting breast cancer compared with those who have had children (via pregnancy, not adoption).
I was quite surprised at this and just had never heard such a thing.
Is there evidence to support or deny a correlation between having children and a decreased risk of breast cancer in women?