"Heredity can flow 'upstream' from child to parent..."

"In pregnant women, fetal stem cells can cross the placenta to enter the mother’s bloodstream, where they may persist for years. If Mom gets pregnant again, the stem cells of her firstborn, still circulating in her blood, can cross the placenta in the other direction, commingling with those of the younger sibling. Heredity can thus flow “upstream,” from child to parent—and then over and down to future siblings...  

The ethics of some reproductive technologies become blurrier in light of the newly complex understanding of heredity’s cross-currents. A maternal surrogate, for example, will likely exchange stem cells with the fetus she carries, opening the door to claims that baby and surrogate are related. If the surrogate later carries her own baby, or that of a different woman, are the children related?"

More at The Atlantic.