"Aborted baby returns 18 years later to save mother's life." Shocking eh? What is more shocking is that it's actually not far from the truth. Researchers from Tufts University and the University of Washington published a report in 2002 describing the case of a woman infected with hepatitis C virus that stopped her antiviral treatment (against medical advice) yet remained healthy. Upon liver biopsy, they found that she had a significant number of male liver cells! By genetically testing these cells and comparing them to those of her four different partners, it turned out that the cells matched partner #1 or #2—men with whom she got pregnant but terminated the pregnancies 17-19 years earlier.

It turns out that this isn't as crazy as you might think. Since about 1996, it has been known that you can find cells in the mother's blood that are clearly from one of her kids—in some cases up to 25 years after they were born. Oddly enough, women who lost a pregnancy actually have more of these cells than women who carry the fetus to term. These appear to be embryonic stem cells that just hang out in the mother's body.

So let's get back to our original story—the woman with the he/she liver. Think about this for a second... or a minute, or whatever. These male liver cells didn't start out as liver cells, they started out as stem cells (which, roughly speaking, are generally blood cells), but as mom's liver came under attack from a virus, baby's cells migrated in, turned into liver cells and went to work. It's impossible to determine if the male cells in this woman's liver are what tipped the balance and kept her healthy, but it sure looks that way. It's not like they migrated to her liver and then sat around doing nothing; once you turn yourself into a liver cell, well, that means you are working for the liver man, detoxifying and cleaning out mom's blood.

Ok, you say, but that's just one woman; she could be a medical sideshow freak. Ah, but she isn't alone. Another study, published in the British medical journal Lancet in 2001, studied 29 women (all mothers with male children) with thyroid disorders and found male cells there also. These researchers noted that the male cells could form thyroid organ substructures that were indistinguishable from the mother's (excepting the Y chromosomes). They come to a slightly different conclusion, suggesting that the fetal cells are actually what is causing the thyroid problems. I don't want to debate that point as it is somewhat of a chicken/egg thing, and it doesn't detract at all from the science fictionish allure of all this.

"Ok Jeff, that's pretty cool, I will give you that. But livers and thyroids? Hardly the stuff of science fiction."

I smile devilishly and wink.

"What? There's more? Well..." (You laugh nervously) "...it's not like these cells get into the brain.. hahahahaha.. haha... ha... ha......... ha."

I deadpan.

"Right, Jeff? Right? I mean," (more nervous laughter, eyes growing wider) "that would be crazy. Tell me these cells don't get into the brain. My god, please tell..."

Fine. I have saved the craziest for last. Yes, Virginia, these cells do get into the brain. Well, into the brain of a mouse at least (a wee bit unethical to be taking brain tissue from a living human). In sum, if you mate a male mouse whose DNA expresses a fluorescing green protein with a female mouse without the green in her genes, guess what? You can find green glowing cells in her brain after pregnancy. God, I wish I could bring Vincent Price back to read this paragraph out loud. Babies exerting mind control by implanting their own cells into mom's brain? Now that is some good science fiction. Yes, you will give me that pacifier (cue Vincent Price laughter).

Note: Lest you think I am sexist, the reason I only cite cases in which the son's cells (and not the daughter's) are found in mom is because most of the literature on this subject is biased that way. It is technically much easier to distinguish a mother's cells from her son's because of the presence of the Y chromosome. The presence of that Y chromosome allows you to physically purify the male cells away from the female ones. I assume that girl fetuses also transfer stem cells to the mother, but this is much harder to address.