This summer I put my Lilkid, as I call him online, on the school bus for the first time ever. Evidently I have “socialized” him enough with other lilkids, because he got on without a backwards glance, ignoring his mother getting all teary and father waving goodbye. He chose a seat and then mouthed through the window with a huge grin, “MOM! I am ON THE SCHOOL BUS! And IT HAS NO SEAT BELTS!!!”

When you have a kid, people tell you various clichés about how your child will be part of you forever. Ladies, in your case, it’s true, and it’s supported by science. 

Thanks to a phenomenon called fetal microchimerism, a mother can carry cells from her fetus in her own body for many years after the pregnancy ends. Particularly in the last two decades, microchimerism has been recognized as the norm rather than the exception. We now know that, instead of being separate systems, the mother and fetus leave a number of permanent marks on each other through the trafficking of cells back and forth over the placenta. Fetal stem cells make their way into the mother’s bloodstream and even into her bone marrow, sometimes contributing to her blood supply for the rest of her life.

Like many parts of having a kid, the consequences of this microchimerism are both good and bad. Fetal cells have been found at sites of injury in the mother while she’s pregnant, or even years later in liver injuries or appendicitis cases, apparently drawn by damage and participating in repair or regeneration. Good news! Fetal cells have also been found in breast cancers much later, again seeming to try and repair the tissue. Thanks, kid!

But the presence of fetal cells is also invoked as the reason why women have more autoimmune disorders, including lupus and thyroiditis, during and years after pregnancy. Immunologists think that this happens essentially because Mom’s immune system eventually realizes that these fetal cells don’t belong to her own body, and attacks them as a result. Hmmm, not great. [However, at least you have some more scientific basis if you hear yourself telling your child, “You are KILLING me!”]

In fact, testing women’s cells for the presence of the Y chromosome — the “male” chromosome, which females shouldn’t carry — uncovers it in about 30% of the bone marrow of grown women and 47% of cardiac aortas. Even among women who have truly never had a reportable pregnancy, 7% or more would test positive for XY cells. Doubling those numbers to account for fetuses of both sexes further supports the idea that many pregnancies go undetected. It’s not just the mothers standing with me at the bus stop who are microchimeric; these problems and benefits apply to more women than we think.

So as I watched Lilkid pull away into a new stage of independence, this geeky scientist thought about how his cells would literally be part of my body forever, for both good and more challenging times. Then the straight Mom kicked in with a host of more mundane worries: “Great — now I have to go look into this ‘no seat belt on the bus’ thing. Did I pack enough snacks for him to eat?” And so on as the school bus drove off for the first of many mornings.

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