it appears a baby's cell tissue gets implanted into the mothers body and remains after birth.

reference:
http://www.nytimes.com/2015/09/15/science/a-pregnancy-souvenir-cells-that-are-not-your-own.html?hpw&rref=science&action=click&pgtype=Homepage&module=well-region&region=bottom-well&WT.nav=bottom-well