Well, there is a general trend: higher education, fewer kids. It's not a 1:1 correlation as many other factors contribute (in particular the higher cost of living; that's an even more important factor if you look at the oddities in South Korea or Japan, and even now in mainland China). Obviously the latter is not "child marriage", but I point at the number of offspring in general.
Japan, China, and South Korea are all very densely populated. Not long ago this was seen as an overpopulation crisis that would cause them to be impoverished. Now the headlines are of a population crash. Neither of these trends is sustainable and you know what they say about unsustainable trends: they won't be sustained. If the population declines sufficiently, say back to the levels before Asian populations grew so quickly, wages will get bid up and houses will get affordable.
I'm not educated enough on the topic to know who is right, but there are plenty of people who say the planet is currently beyond sustainable carrying capacity. Nonstop population growth and urbanization is nonsensical. Who knows what the harmonious number of humans really is, but the only imperative is not to get it wrong in the overpopulation direction and cause billions of deaths from ecological collapse.
One reason I'm not against the attempt of Mars colony is that it would teach us, with a relatively small number of casualties, that humans need a complete ecology around them to thrive.