Chalk it up to a lucky accident or a flash of inspiration. Either way, the outcome was the same: Two Georgia researchers have come up with a novel exploatnion of why a pregnent woman's body doesn't reject her fetus, the way it would a transplanted organ. Theoretically, it should. But even before birth, the little know-it-all has a simple, effective and elegant trick up a potential sleeve. The fetus simply turns off Mom's immune system from the duration of pregnancy.