It remains uncertain whether these cells might be helpful or harmful to mothers, or possibly both, scientists added.

 

Recent findings showed that during pregnancy, mothers and fetuses often exchange cells that can apparently survive in bodies for years, a phenomenon known as microchimerism. Scientists had found that in mice, fetal cells could even migrate into the brains of mothers. Now researchers have the first evidence fetal cells do so in humans as well.

 

The investigators analyzed the brains of 59 women who had died between the ages of 32 and 101. They looked for signs of male DNA ―which, they reasoned, would have come from the cells of sons. (They searched for male DNA because female DNA would have been harder to distinguish from a mother's genes.)

 

Nearly two-thirds of the women — 37 of the 59 — were found to have traces of the male Y chromosome in multiple regions of their brains. This effect was apparently long-lasting: The oldest female in whom male fetal DNA was detected was 94.

The defense system known as the blood-brain barrier keeps many drugs and germs in the bloodstream from entering the brain. However, doctors have found this barrier becomes more permeable during pregnancy, which could explain how these fetal cells migrated into the brains of their mothers.