"Embryologist Magdalena Zernicka-Goetz has spent two decades unraveling the mysteries of fetal development. By studying embryonic mouse cells, she witnessed the embryo's ability to rid itself of abnormal cells as it prepared for implantation in the womb. When Zernicka-Goetz became pregnant at 44, she received a call that took her by surprise: a sample test of the cells in her own placenta indicated that the fetus had trisomy-2, a disastrous extra...