It's not just a "right now" temporary thing. Every country with a high standard of living has birthrates below replacement, often far below replacement. Part of the reason I think is that our economy pushes especially the brightest to extend education into their 20s when people should be working and starting families, part of the reason is that our economy needs to integrate familycare into the expectations we put on workers, and part of the reason is that people are selfish and children and expensive.