It is primarily economic issue but not in that sense. Subsistence farmers tend to have a lot of kids, because each kid expands labour power of the family, improving the quality of life of all its members. In industrialized urban society each kid is a drain on family's resources for the next 20 years and family's quality of life plummets, and then capitalism makes it even worse. We know that even in medieval times birth rates in cities were atrocious, despite minimal women's rights.
It is primarily economic issue but not in that sense. Subsistence farmers tend to have a lot of kids, because each kid expands labour power of the family, improving the quality of life of all its members. In industrialized urban society each kid is a drain on family's resources for the next 20 years and family's quality of life plummets, and then capitalism makes it even worse. We know that even in medieval times birth rates in cities were atrocious, despite minimal women's rights.