Here's how it works, from From: "Astounding: Your Baby Can Heal You!"
"The main study that supports this claim was done using mice.2 In the study, the researchers took female mice that had never mated before and mated them with male mice that had been genetically engineered to make a green, fluorescing protein in their cells. As a result, half of the offspring produced by this mating process also had the ability to make that protein. Since the females did not have that ability, any cells in the mother that produced the green, fluorescing protein must have come from the fetus developing inside her.
"Now, of course, this study didn’t involve humans, but its results neatly explain a puzzling fact that has been known for quite some time. In the authors’ own words:
" 'These studies were inspired by the recovery noted in peripartum cardiomyopathy [a heart problem that usually occurs in the last month of pregnancy], whereby a remarkable 50% of women recover from heart failure spontaneously. Peripartum cardiomyopathy has the highest rate of recovery among all etiologies of heart failure, and the reasons for this high rate of recovery are not understood. In fact, it was this very observation that prompted us to hypothesize that there might be a fetal or placental contribution to counteract maternal cardiac injury.'