Baylor College of Medicine researchers report women who have their first child before the age of 24 experience the most dramatic reduction in breast cancer risk.

The relationship between age of first pregnancy and the incidence of breast cancer, however, doesn't establish that an early pregnancy protects against cancer and no mechanism for such a protective effect has been found, the scientists said.

The Baylor College researchers studied the number of breast stem cells in mice giving birth at an early age and compared that with breast stem cells in mice never giving birth. The researchers found mice having an early pregnancy had half the number of breast stem cells found in mice that never gave birth.

"Stem cells are long-living cells," said Professor Yi Li, one of the researchers "One theory is that they can thus accumulate more mutations and are probably the most susceptible to giving rise to breast cancer."

The study appears in the journal Stem Cell.