Poorer countries need more labor workers, which big families help with that. 


First World Problems...

Then we look at the poorest countries and they have a lot of children for each couple.

riberet19

That is true. 
The problem is even the couples that are deciding to have babies only have one. 
Cost and education is so expensive there. 

Satosora

But in most cases it is needed for the birth rate to grow.

Yes, it is clear that many things are happening, this is just one of them.