Read a SF novel recently [0] where the rejuvenation treatment that you take periodically to extend your life indefinitely also makes you sterile temporarily. If you chose to have children (which is socially frowned upon), you must stop taking the treatment forever.<p>Not sure that we would be able to implement such a rationale solution, but at least it was pretty logical. The novel is essentially about the other grim decisions you have to take to have a constant population.<p>[0] This was in a book with multiple short stories from Paolo Bacigalupi. I believe this is Pump Six and Other Stories.  Most of the novel are rather pessimistic, but at the same times sounds plausible enough to happen. I also recommend The Wind Up Girl from the same author.