ScienceDaily (Sep. 26, 2012) — Male DNA is commonly found in the brains of women, most likely derived from prior pregnancy with a male fetus, according to first-of-its-kind research conducted at Fred Hutchinson Cancer Research Center. While the medical implications of male DNA and male cells in the brain are unknown, studies of other kinds of microchimerism -- the harboring of genetic material and cells that were exchanged between fetus and mother during pregnancy -- have linked the phenomenon to autoimmune diseases and cancer, sometimes for better and other times for worse.
So fetal cells even cross the blood-brain barrier. If you had asked me earlier, I would have said that I can't imagine that's possible.
Here's a previous post on microchimerism.