During pregnancy, fetal cells enter the mother (and vice versa).  They linger in the mother for decades at minimum – probably for life.  The first thought has been that this might be the cause of higher levels of autoimmune diseases in women, and that might be so.  But there are more interesting possibilities.

There is evidence that fetal cells may be beneficial.  Experimental work in mice shows that a sub-population of fetal cells (pregnancy-associated progenitor cells, or PAPCs) have stem-cell-like properties.  They show up at injury sites, and there are some indications that they contribute to tissue repair.  For example, after heart damage in  mother mice, fetal cells engraft in the heart and differentiate into cardiomyocytes.  They cross the blood-brain barrier and apparently differentiate into neurons.

We now know that fetal cells also enter the brain in humans, but we do not yet know if they act as neurons there.  But that’s the way to bet.

The question is, what acts by these fetal cells would be favored by natural selection?   Aiding Mom’s heath and survival would obviously be favored:  that would allow her to care for the child that threw off those fetal cells, which would increase the kid’s fitness.  This would be true even after menopause, in humans.  Which raises the question: what can those fetal cells do that Mom can’t do for herself?  Well, for one thing, they’re younger, have longer telomeres, have fewer somatic mutations.  Maybe they’re good at repair.  If they’re contributing to immune defense, well, they bring in different HLA alleles.

Of courser, they have their own genetic interests, which sometimes differ from those of the mother.  The kid would benefit if Mom somewhat changed her ways in the direction of investing more in that child (acting as if it was twice as important as other full sibs) and if she had her later children with the same father:  such children are twice as closely related to the shedder than stepsibs. That may not be the optimal course for the mother’s genetic interests, especially if the father is something of a loser – but the increased relatedness of full sibs would usually trump that, from the fetal cell line’s point of view.

Of courser the fetal cells of those later children have their own interests  and will push in different directions.  If these fetal cells can recognize the degree of kinship in future pregnancies, life gets complicated.

Since some women have only had male children, their fetal cells are easy to recognize (they have a Y chromosome)  and I suppose we could figure out a way to zap them,  possibly returning their mother’s brain to its original unmanipulated state.  Except for Toxo and advertisers and whatnot.  Feminists take note: if you have ever been pregnant, you’re not the same person anymore. With any luck, we will soon see a new version of Morgellons syndrome, with genetic formication.

One fun possibility is that some of these cells are passed on, generation after generation – from mother to a female fetus , and so on indefinitely, rather like mitochondria.  It is easy to show that such cells would have to have a positive effect on female fitness.  We know plenty of similar examples of symbiotic bacteria in insects – here, we’re talking about a symbiotic (technically, mutualistic) cell line that started out human.  Such a symbiotic cell line would co-inherit with a particular type of mitochondrial DNA: if it conferred extra fitness, that mtDNA haplotype would expand rapidly.






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13 Responses to Insidekick

  1. That Guy says:


    Do you mean that the exogenous cells are passing around in the bloodstream of the mother, or that they form tissue in the mother’s body itself?

    • gcochran9 says:

      Apprently they take up permanent residence.

      • That Guy says:

        If that’s the case, then a cluster of them could form new nueronal tissue in the mother’s brain, and if that were possible, would it mean that women who gave birth over time start to exhibit changes in mental function… like becoming subtly more rational?!

  2. AllanInPortland says:

    WNYC’s Radio Lab did a reasonable, if slow & obvious, show the topic:

  3. The fourth doorman of the apocalypse says:

    It would seem that the father’s genes can have an effect on the mother regardless of whether the child is an XY individual or an XX individual … and that selection might favor those fathers who have somatic genes that express themsleves in the mother’s brain tissue …

  4. Steve Sailer says:

    Does this have anything to do with lower breast cancer rates in women who have been pregnant, or is that something else entirely?

  5. Dahlia says:

    Skip to 1:51:38 to hear Paul Ewald discuss breast cancer and pregnancy.

    Dr. Cochran,
    I was a participant in a yet-to-be published that touched on autoimmune diseases
    in pregnancy (and stress) and the woman in charge was pretty knowledgable about fetal
    stem cells and the latest theories as well.
    With both my boys, and only them, I was struck by post-partum thyroiditis. I was
    in the control group because I tested postive for antibodies of some kind (don’t remember
    the specifics). Both times, my memory was profoundly affected, but even more so the
    second time. My memory is making a comeback, but progess is slow. I lean toward
    the belief that the stem cells are helping.
    How my pregnancies affect me seems to depend so much on how masculine versus feminine my children are and thus how much testosterone or estrogen they added to my system. Cholestasis of pregnancy only occurred in me with my most feminine daughter, the other girls produced cholestasis-lite pregnancies. With the boys, I felt great when I was pregnant with them. The post-partum period was the killer with far lasting consequences.

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  7. pablo brandt says:

    That may be the reason why low life people get so much pregnant, despite not having an adequate background for a new breed and with available anticonception methods and free abortions. The boost from the fetal cells. Like some zombie parasites. Is this Halloween again?

  8. I am for stem cell research in healing injuries, diseases and even growing body parts. But when you start cloning things and using stem cell for reproduction we should ban that because that can cause all type of problems down the road

  9. Pingback: Enmadrarse, desmadrarse y comadrarse: lo sublime y lo monstruoso de la maternidad - Disidentia

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