Everywhere except sub-Saharan Africa and parts of the Middle East have sub-replacement birthrates at this point. Including India and China. China has started seeing contraction, India will start seeing contradiction in ~20-30 years since the measures lag.

It is by no means an issue just in the West.

You're right the situation is different with respect to Nigeria, but the birth rates are also falling in all of the remaining countries. Nigeria's is still high but also falling.