Into the weird part of biology....

Microchimerism: the state of having tiny amounts of another person's cells functioning in your body.

Microchimerism has now been found in the brains of mothers and has been linked to lower levels of Alzheimer's. The study focused on women who had male cells inside them because women have two X chromosomes and men have an X and a Y chromosome, so it's easier to find a male cell in a female, just watch for the Y.

It is currently unknown as to why there is the correlation with Alzheimer's as the field is far too new for much to be said definitively.

For more information: The Scientist: Swapping DNA in the Womb