India, egypt, libya, latin america are poor, yet their birth rate is going down. Africa would probably start to shrink later too in the same way as Europe after 19th century. Children per fertile woman is going down, but antibiotics and vaccines saves most children so their population is rising for now.

Also if this trend continues we won't probably see china in a half century as a state anymore. They overcount people a little for like 200 million that also mostly were "born" in 80s (need verification), which also shrinks the working people. With 1.0 children per woman, in a century 1.3 Billion is halved towards 0.6 billion. With overcount it would reach this level much faster, and before reaching China would collapse through pension burden. They could refuse to pay pension, but spending would fall, so the GDP.