Mothers can carry their children's cells for years after giving birth. It's unclear whether these cells are a health hazard:
Fetal cells seem to help out with bad livers, but show up at arthritis and rheumatism sites and look like they might be on the attack. There are hints that cells can go both ways, helpful, then hurtful.
What produces these behaviors? Dr. Johnson sighed and said the list of explanations keeps getting longer. It could depend on the disease. It could depend on the type of fetal cell (they can be different). It may depend on the mother's age, how many conceptions she's had, or who the father was. (Fathers can be genetically similar to the mother or genetically very different, and that variance seems to matter.)
A new RadioLab episode digs deeper into the science.
The Dish 