This is a question that statistics can answer. I don’t have the data myself but I’ve seen one analysis that pins most of the drop in birthrates to childlessness rather than fewer children per mother. That is, among women who become mothers, the average number of children is relatively stable over time. But the proportion of women who
never have a child is rising rapidly.
I’ll add that the drop in birthrates is not something that has persisted for generations—as recently as 1950 Japan had a fertility rate over 3.