Frustratingly enough the report, perhaps being preliminary, does not break it down by socio-economic status. But going off of what I remember from articles I read in grad school (did some research on aging population and declining birthrates), broadly speaking and in general, and at least in the US, it is not the poorest of the poor who are making all the babies. It is higher socio-economic strata that are having kids, especially multiples, because they can afford it and if I recall correctly, sometimes the kids are treated as status symbols.

Mini-rant: what this article left out was that 1. birth control is more widely available in the US (at least for now) and 2. women since the 70s have increasingly had more of a say in family planning and when women have more control of family planning they tend to dramatically reduce the number of kids they have, especially when it is not economically feasible to have them, regardless of socio-economic status. I really really wish the article had said something about that. For example, birth rates are falling in India and beginning to fall in Africa as well as Latin America and it positively correlates to increased autonomy among women. Grant women the rights they are due and birthrates fall.