Middle income countries like China have low fertility.
Lower-middle income countries like India are around replacement level (I think officially it's lower than replacement but lot of births are "unofficial").
West African fertility rates are high... but A LOT lower than they were 50 years ago, and still trending downwards.
Add the mortality and disabilities later on. Overall I mean that once you don't have to have 4+ kids to assure at least 1 can take care of you later in life, that is great for lowering fertility rates.
Middle income countries like China have low fertility.
Lower-middle income countries like India are around replacement level (I think officially it's lower than replacement but lot of births are "unofficial").
West African fertility rates are high... but A LOT lower than they were 50 years ago, and still trending downwards.
Whatever the reason, it's happening everywhere.