During pregnancy, cells sneak across the placenta in both directions. The fetus’s cells enter his mother, and the mother’s cells enter the fetus. A baby’s cells are detectable in his mother’s bloodstream as early as four weeks after conception, and a mother’s cells are detectable in her fetus by week 13. In the first trimester, one out of every fifty thousand cells in her body are from her baby-to-be (this is how some noninvasive prenatal tests check for genetic disorders). In the second and third trimesters, the count is up to one out of every thousand maternal cells. At the end of the pregnancy, up to 6 percent of the DNA in a pregnant woman’s blood plasma comes from the fetus. After birth, the mother’s fetal cell count plummets, but some stick around for the long haul. Those lingerers create their own lineages. Imagine colonies in the motherland.
Moms usually tolerate the invasion. This is why skin, organ, and bone marrow transplants between mother and child have a much higher success rate than between father and child.
And in terms of fetal cells and what they can do for the parent who carried the child:
It turns out that when fetal cells are good, they are very, very good. They may protect mothers from some forms of cancer. Fetal cells show up significantly more often in the breast tissue of women who don’t have breast cancer than in women who do (43 versus 14 percent). Why is this? Fetal cells are foreign to the mother because they contain DNA from the baby’s father. One theory is that this “otherness” stimulates the mother’s immune system just enough to help keep malignant cells in check. The more fetal cells there are in a woman’s body, the less active are autoimmune conditions such as rheumatoid arthritis and multiple sclerosis. These conditions improve during pregnancy and for some time afterward — suggesting that the mother’s immune system is more focused on attacking the “other,” not herself. There’s also tantalizing evidence that fetal cells may offer the mother increased resistance to certain diseases, thanks to the presence of the father’s immune system genes. These are new weapons in the war chest.
Of course, pregnancy involves a lot more than just the exchange of cells and any benefits for either body. But it’s a fascinating process nonetheless.