The birth rate is already decreasing. It has been for many decades. This is greatly tied to wealth. As a country becomes more wealthy, the birth rate tends to decrease. In the past, it was more important to people to have more children. Not that long ago, the majority of society was involved in farming. Now it is just a small percent of the population. When people had old traditional farms it required a lot of work, so people would have more children to have more free labor. This is still the case in some places, but it's happening less an less as technology relieves farm workers around the world.