Stem cells may solve mystery of early pregnancy breast cancer protection.

The answer to why an early pregnancy seems to protect against breast cancer could rest with a decrease in stem cells found after animals have given birth, said researchers at Baylor College of Medicine in a report that appears in the current issue of the journal Stem Cell (see also Baylor College of Medicine).

Women who have children young, at least before the age of 30, reduce their risk of developing breast cancer, said Dr. Yi Li, a professor in the Lester and Sue Smith Breast Center at BCM. The most dramatic reduction in risk occurs in women who have their first children before the age of 24. However, the mechanism by which these early pregnancies provided protection has proved elusive.

The promise of such work is important.

"If we can figure out the mechanism behind this, we could develop a pill that we could offer young women in high school and college that could significantly reduce their risk of breast cancer," he said.

However, he said, there are many steps to be taken before he and his colleagues can determine how best to do that. Understanding why stem cells decrease in women who have their children young could prove an important advance.

In studies in mice, Li and his colleagues compared the numbers of mammary or breast stem cells (early cells that can differentiate into breast tissue) found in mice that had had babies at an age equivalent to the teens to mice that had never had babies.

Using proven scientific techniques, they found that the mice that had ...