As standsrds of living rise, people tend to have fewer children. Some of the most developed countries actually have decreasing populations --- there are several with negative population growth in Europe. The US has nearly zero population growth due to birthrate --- it continues to grow through immigration.

If you live in a very poor country, children are your economic security, because they can go to work to help bring money into the family and they can support you in your old age.

If you live in a wealthy country, children are costly because you have to raise them in an expensive society and do things like pay for braces for their teeth and put them through college. And if your country has something like Social Security, or you earn enough to fund your retirement, you don't need children as much to see you through your old age.

If the rest of the world develops more economically, then maybe population growth will stabilize. Of course, if the rest of the world develops more economically, they will also want more consumer goods --- fewer people, but each one consuming more resources.