By Malcolm Ritter
The Associated Press

    Moms, as you reflect on Mother's Day today, here is news just for you: Even if your kids have grown up and moved away, they probably still carry a part of you with them.
    But not in the way you might think.
    What's more, you probably still carry a part of them.
    It turns out that even decades after a woman gives birth, she can still have cells in her blood and tissues that came from her children during pregnancy.
    And by the same token, many adults appear to harbor such cells they picked up from Mom during their time in the womb.
    As a recent editorial in a pediatrics journal put it, "So you think your mother is always looking over your shoulder? She may be IN your shoulder!"
    The big question for scientists now is whether harboring these foreign cells is good, bad or irrelevant for a person's health. Some evidence suggests they may set the stage for several kinds of autoimmune disease, in which the body mistakenly attacks its own tissues. But some scientists suspect they may also be helpful.
    Actually, these foreign cells aren't the ones that slipped from fetus to Mom, or vice versa, during pregnancy. Rather, they are descendants of stem cells that transplanted themselves, took root and started pumping out the progeny found decades later.
    The result is a vanishingly small dose of foreign cells in adults. One study, for example, found up to 61 fetal blood cells per tablespoon of blood from women. That's less than one in a million, notes J. Lee Nelson, a key figure in the young field of studying the phenomenon. Harboring a small dose of fetal cells from a long-ago pregnancy is "definitely very common in healthy individuals," said Nelson, an autoimmune-disease specialist at the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle. The great majority of women who have had pregnancies -- even those ending in miscarriage or abortion -- probably carry detectable fetal cells, she said.
    As for adults carrying cells from Mom, Nelson said a recent study found them in the bloodstreams of almost a quarter of women tested. She said she suspects the proportion will be higher when more extensive studies are done. In fact, she thinks "it's at least not at all uncommon, and the probable answer is . . . that most of us have a low level."
    The foreign-cell phenomenon is called microchimerism (MY-croh-KY-mer-ism). Scientists using genetic tools to identify such cells find them repeatedly in sick and healthy people. One research team found male cells in women's bloodstreams up to 27 years after they had given birth to a boy. On the other side of the coin, Nelson and colleagues found that adults in their 40s still carried blood cells from Mom. Of course, women who have been pregnant could have cells from their own moms as well as their children. And sure enough, last year scientists documented that a deceased woman in her 40s harbored cells from her children, born six and eight years before, and her own mother.
    One thing about foreign cells is already clear to Nelson: They are not any reason to blame anybody for disease. "It's not an issue of blame. These are rogue cells deriving from the pregnancy," she said.
    Quite the contrary. Judith Hall of the University of British Columbia recalls lecturing at a medical school about adults carrying cells from their mothers. Afterward, she said, a young woman came up to her with an extraordinary reaction.
    "My mom died about five months ago, and I've had the hardest time adjusting to her not being around. Now she's here," Hall recalls the young woman as saying. "It really was very touching."