There's a really outstanding SF/Fantasy book called
Replay by Ken Grimwood that discusses this idea at length. A guy has to go back and basically relive his life multiple times, and he makes different choices each time and we see the paths those different choices lead him on. In one of his replayed lives, he figures out how to have a perfect life, and has wonderful children and everything is great, and then when he has to relive his life again, those children are gone. He can never recreate them because even if he marries the same woman, they can't make sure the same sperm fertilizes the same egg to make the same kid he had loved so much. So he never has kids again in any of his later replayed lives, because it's too painful to lose them.
Great book.