I can’t prove it but I suspect that they are having about as many children as they want and our expectations of ‘fertility rates’ are actually skewed by the number of unwanted pregnancies that were forced on people who then existed in the space of ‘We didn’t ask for this but now we love the little shit so I guess we’ll make the best of it.’ The world is and has been changing so fast for the last century or so that our sense of long term trends is much harder to understand.