Dragon's Time (Dragonriders of Pern, Bk. 23)

Even though Lorana cured the plague that was killing the dragons of Pern, sacrificing her queen dragon in the process, the devastating disease has left too few dragons alive to fight the fall of deadly Thread. As morale at Telgar Weyr plunges in the wake of the widespread loss, and Weyrwoman Fiona and the harper Kindan struggle to keep hope from vanishing, a pregnant Lorana decides she must take drastic steps. Nothing short of manipulating time itself will undo the damage done by the plague. But attempting time travel could have devastating consequences--altering history, and destiny, forever. Now, somehow, Lorana must find a way to accomplish what seems impossible: bending the natural laws of the universe without wreaking havoc on existence itself. To ensure the future of Pern, she's willing to take the fateful chance--even if it demands yet another, far greater, sacrifice.