Katherine Rowland in Aeon:
Within weeks of conception, cells from both mother and foetus traffic back and forth across the placenta, resulting in one becoming a part of the other. During pregnancy, as much as 10 per cent of the free-floating DNA in the mother’s bloodstream comes from the foetus, and while these numbers drop precipitously after birth, some cells remain. Children, in turn, carry a population of cells acquired from their mothers that can persist well into adulthood, and in the case of females might inform the health of their own offspring. And the foetus need not come to full term to leave its lasting imprint on the mother: a woman who had a miscarriage or terminated a pregnancy will still harbour foetal cells. With each successive conception, the mother’s reservoir of foreign material grows deeper and more complex, with further opportunities to transfer cells from older siblings to younger children, or even across multiple generations.
Far from drifting at random, human and animal studies have found foetal origin cells in the mother’s bloodstream, skin and all major organs, even showing up as part of the beating heart. This passage means that women carry at least three unique cell populations in their bodies – their own, their mother’s, and their child’s – creating what biologists term a microchimera, named for the Greek fire-breathing monster with the head of a lion, the body of a goat, and the tail of a serpent.