Mother's day is nigh—sort of. Anyway, without knowing it, you might have already given your mom a pretty lasting gift. But whether it helps or hurts her—or both—is still an open question. In this Radiolab short, Robert updates us on the science of fetal cell—one of the first topics he covered as an NPR science correspondent.

Six years ago, wearing his NPR science-correspondent hat, Robert presented listeners with this question: what if we told you that legions of fetal cells hang out inside a mother for decades after she gives birth—and might even help heal her when she's sick or hurt? Back then he described this as a "too dangerously beautiful idea" for the scientists researching fetal cells. They wanted to believe it, but the evidence wasn't there yet. One of those scientists was Kirby Johnson at Tufts University, who explained that the cells might also hurt the mother. He wasn't sure which. "I think that that's something that we're going to see within the next five years or less," Kirby said. So Robert thought it was high time to call Kirby for an update, and to ask once again about Kirby's personal stake in the work he's doing.