A study was conducted in Norway and it's results published in 2009. Here is a synopsis.
Researchers looked into the possibility that hCG (a prevalent pregnancy hormone) provided protection against breast cancer for the child-bearing mother. Compared to 'normal' pregnancies, 'Down Syndrome pregnancies' yield higher-than-normal hCG levels from late 1st trimester to the middle of the 2nd trimester. The study hypothesized that women who gave birth to children with Down Syndrome (DS) might have a lower risk of breast cancer than women who gave birth to children without DS.
The study examined all mothers of live-born children in Norway and Sweden from 1967-1973 through 2004. The study period yielded the following results:
54,063 women developed breast cancer.
5,330 children were born with DS.
139 breast cancer cases were diagnosed in the mothers of DS children.
Adjustments were made for relative risks, risk factors, etc.
In the final analysis, results indicated that mothers of children with DS were at a 23% increased risk of developing breast cancer. The increased risk was limited, however, to women who had a child with DS after age 30. It also seemed to be confined to women whose cancer was diagnosed before the age of 50.
The original hypothesis that exposure to elevated levels of hCG may provide maternal protection against breast cancer was NOT supported.
An interesting addition....
In women with Down Syndrome (not the mothers), reported studies suggest that breast cancer is nearly 10x LESS frequent than in the general population.