Lina Zeldovich at Undark:
A biological phenomenon, microchimerism refers to the presence of a small number of cells from one individual within another genetically distinct individual. It most commonly occurs during pregnancy when fetal cells escape into the mother’s bloodstream or maternal cells sneak into the placenta, eventually becoming part of the embryo or fetus. Likewise, twins may exchange cells before birth, too.
The cases aren’t that rare. “Approximately 8 percent of fraternal twins and 21 percent of fraternal triplets carry blood cells from their companions in utero,” Barnéoud writes, citing a 2020 review. Similarly, fetal cells that wander outside the placenta can persist in the mother’s body for years, genomic scientist Diana Bianchi discovered decades after Mrs. McK’s case, in 1993. Bianchi and her team found male cells in the blood of six women who had given births to sons from one to 27 years earlier. Male cells are easier to spot in women because they have X and Y chromosomes in their cell nucleus while female cells have two X chromosomes, and the Y chromosome stands out. But males can carry foreign cells too.
More here.
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