Fertility has already fallen below replacement levels in most of the world. The only exception is Africa however carbon emissions from that part of the world are so small it barely makes a difference. Even if you wanted to focus in on Africa you will find the countries with the highest emissions like South Africa have a fertility rate barely above replacement level.

The reality is nearly all the increases in population are built in already, it's caused by people surviving into old age. The population is going up and nothing can be done about it.

This is why arguments about population are pointless with regards to climate change, we resolved that years ago through education and better health provision.